If the kid has SPD and ADHD, he is not going to change. It's only going to get worst as he get older.
If you can't deal with it, and nobody will think any less of you. You have to leave.
Seams like nobody is doing anything or cares to help, you should not have to do it on your own

your DH disengages from his own son and leaves you to deal with it?

No.

YOU disengage, and let DH deal with his own son while you deal with your bios.

You need to stop enabling your husband. Stop being his sons primary caregiver. He needs to do it. Trust us.

My cousins son has ADHD. He was diagnosed at a young age. Medication and diet helped. But I can tell you that he was still so impulsive that you just couldn't trust him. As he got older he was a follower and made a lot of bad choices. I could tell by looking at his eyes if he had meds or not. He does know right from wrong and he wanted to be good and he has a good heart but it seemed like he just couldn't help himself. I would start with getting a proper diagnosis then take it from there. Also watch his diet and don't give him too many choices as far as toys to play with or picking out what he is going to wear. And you are right, the other kids will keep their distance from him. How is he in school?

OP said:
"...it sounds horrible, but it's one thing if it was my bio kid that had issues to work through, but it's so much more effort to want to deal with it with SS."

I think it's pretty typical that it's harder to do this with a SKid because you're ALSO having to deal with your SO, who isn't going to be as objective as an outsider (we're all that way, it's very natural, really!). Many times, our SOs get defensive of their precious BioKids because they take any critical commentary as if you just shot their dog.
In the front yard.
With their kids watching.
In the rain.
A cold rain.

I dramatize.

Anyway, problem is, the fulcrum in this is your husband. He needs to be the lever, and the SITUATION needs to be the fulcrum. Point out to him that certain behaviors, whether exhibited by your kids or his own, or even total strangers, is UNACCEPTABLE. The best way to change those behaviors is to point out the bad result, then work your way back to point out the cause in a cause-and-effect exercise, but don't mention names because it's a behavior you're trying to make them aware of, and to show them how changing that behavior will make everyone's lives better. Support your husband as he leverages the situation.

Oh yes. The infamous pre-visitation behavioral degradation and post visitation detox. I remember it far from fondly.

I took the zero tolerance position and gave the Skid no quarter in complying with our family and household rules regardless of what the case may have been in SpermLand during visitation. The 10-ish days before visitation and the three-ish weeks of post visitation detox were still a challenge but the onus was on the Skid to extricate his head from his butt or suffer the consequences of his chosen inappropriate behaviors. We lived this in one form or another for 16 years under our Custody/Visitation/Support CO until SS aged out from under the CO on his 18th B-day. Sadly the SpermClan banished him at that point. Once they were no longer forced to pay to play (CS) they wanted little to do with SS except to try to guilt him into sending money for the support of his three younger also out of wedlock SpermIdiot spawned half sibs. Though their crap never ceased to break his heart he wrote them off shortly after his last COd visitation.

Interestingly now at 25 he doesn't remember those struggles around visitation on our end of that equation. He remembers the toxic crap in SpermLand clear as day though.

A couple of months before his 23rd B-day he asked me to adopt him and to take our family name. We made that happen. It was a cathartic purge of the shallow and toxic end of his gene pool and he hasn't looked back.

As infuriating as this type of situation can be for the adults, sadly it is the kids who bear the biggest burden of any toxic gene pool issues.