You mention autism testing but this sounds like a deeper issue.  I"d be looking for a good therapist right away.  

I'm so sorry you're going through this, I think it's unfair you all have to live this way, and it must be so hard being pregnant and having a LO while dealing with this. The biggest problem you're going to have is if your ss can't move out at 18 because of these issues. Dh needs to put all his time and energy outside of work into getting this kid ready to launch, even if ss is renting a room and working a minimum wage job for eternity. Therapy, psychological assessment, camps, group sessions, volunteering, clubs. SS needs to feel like productive member of society and learn to fit in. And the more you get him out of your house the more peace for you, but just make sure your dh is doing all of the running around for ss not you. If you are financially able to you could always sell the home you have now and get two smaller ones, so you'd have a place to take bios and raise them away from ss. Dh might not like it, but it would just be temporary until ss becomes an adult. You'd be able to go over and all be a family when it works instead of being on high alert to protect children 24/7.

He's not normal.  School mental health support most likely not good,  He needs really good doctors.  Or really good lawers 

you can not have him living with you and your bio babys

I'll cut to the chase. Your DH is an idiot.  You need to tell him that his failed family parenting mistake is no longer welcome in your home or near you and your young ones.  This kid needs a life of such escalating misery hell that he will start twitching and sobbing if he even thinks about misbehavior or being a bully.

It is long past due for a peer of this kid, one he tries to bully, to beat this kid into a simpering puddle of bloody protoplasm.  As someone who was a target of bullies starting about the same age this violent POS spawn of shit parents is I have zero tolerance for their crap and for the ball-less no consequences crap in schools these days.  

I solved the problem when I came to the epiphany that getting bullied and getting hit hurts whether I am fighting back or not.  At that point I committed to myself that if a bully was going to attempt to bully me that I would perpetrate overwhelming immediate violence on them to the point that they would carry the reminder of their poor choice for life.  Indelible facial scars, living live with only one testicle, whatever it took.

Interestingly it only took one disfigured bully per school to end any further attempts to bully me at that school.

This was also the stage in life that I arrived at not giving a shit about the why they did what they did. I cared only that they did it and that I could ensure they never chose to do it again.

The first major response that I took occurred in 8th grade when a kid I had gone to elementary school with decided it was a good idea to jump me from behind knocking me down on my hands and knees in the couryard outside of the lunch room in Jr. High.  It ripped the knees out of a pair of brand new jeans, tore up the palms of my hands.  As I stood up one of his younge minions did the same after running across the courtyard behind me. As I stood up the second time I pivoted towards him and hit him in the middle of his bottom lip after driving off of my legs as I got up.  His face exploded, his bottom lip was bisected hanging in two flaps down either side of his teeth, several of his teeth were snaggled and very loose, and the entire front of his shirt was soaked in blood in a matter of a few seconds.  I then pivoted back to the first guy who put his hands up and backed away then turned and ran.

I was called the  the Principal's office. He had my file on his desk when I arrived.  The first thing he said to me was "Who are you?". He then did a quick recap of that I had been in his school for two years, had decent grades, and been in no trouble, and all of my teachers had good things to say about me. I was a decent student, easy going, a decent athlete, had a few friends.  We were from the invisible middle of the social structure.  The Principal asked me what had happened. I explained. He saw my bloody knees and palms and the splatter of blood all over me from when I exploded the face of the second guy.  I got in zero trouble. Both of the bullies were suspended for 2wks and had to attend summer school.

About that same time I had a bully in the neighborhood as well.  A kid I had been friends with when we first moved to that town had become a minion of a much older kid who was on his 7th year of HS.  They decided that I needed my ass kicked.  My parents were out of town for a few days, my younger brother was staying with a neighbor.  I heard something outside and went through the house looking out the widows. I saw the two of them behind a row of hedges in our front yard and heard them outline how they were going to get me out of the house so the older dumb-ass could kick my ass. I decided I was not going to hide. I went out the front door, sat on my brothers small bicycle and the kid who was my friend came up to me to "talk" obviously keeping my attention to the front, at the last minute I stood up, grabbed my brother's bike and slammed into to the face of the older guy who was at least 6+ years older than I was.  I commenced beating the snot out of him with the bicycle.  He took off running all cut up.  

Not the last time that  happened at school Though it never again happened in a neighborhood.  I was in 3 Jr. High Schools. The first for 1.5yrs then one for the last half of 8th and first half of 9th grade and another for the last half of 9th grade.  The second Jr. High had a bit of a pecking order welcoming committee for the new boys when they arrived.  First was getting surrounded by all of the boys with some yelling, shoving, and threats. Nothing shoving back harder putting a couple of them on their asses did not end.  Then my screwed up big time.  Phys Ed that term was swimming.  I was a highly successful AAU level swimmer.  During a pool session after practice they surrounded me and decided it would be fun to rough me up shove and hold me under.  

Bad move.  Instead of resisting them on the surface, I went under and started punching nut sack after nut sack, viciously grabbing and attempting to remove testicles. There was screaming, puking, etc... from several of them.  A few of them went to the clinic in ambulances.  Everyone of them walked around school like they had elephant testicles for a week or so.  A couple of them had a ruptured testicle. A couple had to get stitches in their scrotum.

No more bullying at that school.  I got in zero trouble as there were any number of witnesses.  Mainly the female classmates.

Coddle parenting creates bullies IMHO.  Confident parenting prevents it or ends it. Either by giving their own bully spawn consequences that ensure the kid will never again pull that crap. Or, when the children of the product of quality parenting destroys the bully when the bully pulls their crap.

I hated fighting. I still do. It scares the crap out of me.  I had been bullied over the couple of years before I exploded the bully's face.  My parents made it clear that if I did no deal with the bullies myself, that I would deal with them when I got home. Dad and I started Kung-Fu school then.  I also got the 1:1 USMC dad instant destruction of an attacker lessons via overwhelming destructive violence.

Very effective methods for dealing with a bully or bullies.

Dad needs to put his foot up this kid's ass rather than failing to "deal with it" before one of this kid's targeted victims beats him bloody leaving live long physical and psychological scars.

Which IMHO can't happen soon enough.

My SS was also a mellow easy going kid.  In 2nd grade he was targeted by a 5th grade kid on the school bus. SS would come home with bruises on his neck.  The kid would sit behind him on the bus and choke him. Both on the AM ride to school and the PM ride home.  When  SS started crying about going to school and we noticed the bruises SS told us what had been going on. We called the school, the director of bus operations, etc... It did not stop as no one apparently saw anything.

After a couple of weeks of nothing from the officials SS and I worked on a solution.  I put two dining chairs one behind the other. I sate behind him and we talked through what the kid was doing.  I coached that when the kid grabbed him round the neck that SS should pull his feet up on the seat, scoot up the back of the seat so he was close enough to reach back and grab the kids ear, the dig his nails into the base of his ear, squeeze as hard and he could then pull and tear until the ear came off.   SS cried that he did not want to hurt the kid. I asked if the kid cared about hurting him.  A few weeks later... the phone rang from school SS had partially severed the kid's ear on the bus.  We went in with our lawyer, pics of SS's bruised neck showing freshened bruising repeatedly over time, information on the calls and meetings with the school and bus administrator, and the clear message that if they thought SS would experience any consequences it would be all over the media and in the court room before they could get the words out of their mouths.  That night there was a knock on our door. The bully's dad was on our front step with the ER bills on getting the kid's ear stitched up.  His kid was a few feet behind him.  He tried to intimidate me. Nope. I called SS to the door, showed the dad the bruises on my much younger smaller kid's neck, had SS tell the dad what had been happening, and then told him if he did not get off of my property I would press charges against his kid.  That poor kid was dragged down the street back home by the scruff of his neck while his dad chewed his ass and put a side foot to his butt every several feet.

End of SS getting bullied.

Yes, I  have issues with bullies. They need to be ended as early as possible and as brutally as necessary to destroy any future thoughts of bullying. I don't give a shit why they do what they do.  I do not care that they are CODs, or that they were bullied themselves, or that they have whatever influencing syndrome of the moment is couture in the excuse  based pseudo science industry.

If your DH won't do it, I hope that your idiot SS-13 meets an intended victim that will knee his nuts out of his mouth, bust his face into reconstructive state mush, or whatever other overwhelmingly painful injury sends the message that he is the POS that he is.

Grrrrrrrr.