It sounds like you have a decent amount of time where you are in charge of him when his dad isn't home.  I always maintain that if you trust me to be in charge of your child.. you trust me to make good decisions about how to deal with him when I am the one there.  You don't get to second guess my actions.  If you don't trust me to care for your child.. find someone else to do it. 

That being said.. I do NOT advocate ever laying a hand on someone else's child.. but other discipline should be not a problem.

So, when the kid back talks you.. refuses to do his homework or refuses to clean his room when you are there with him?  You deal with it.  I would be making it very clear that "I'm the one here with you now buddy... your dad may come in later and rescue you.. but you are under MY thumb right now and you WILL do what I say... or else.  You may not like it, but while you are here, I am in charge.  We can do this the easy way.. or you can make it ever so much harder on yourself.  Personally, I prefer the easy way.. I dont' think your a bad kid, but you are acting like a brat and that's not a good look on you kid"

Some good consequences?

1.  Cut off internet

2.  Confiscate games and phone and computers

3. Trash bag the mess in his room.

4.  Remove his TV (if he has one.)

5.  No visitors allowed.. No access to any phone.. nothing.

Now, for your husband.. after you explain that he will NOT be second guessing consequences for things that happen while he isn't around.. because that undermines YOUR authority.. you tell him if that won't work for him.. then he can hire an after school sitter for the kid.  Tell him that you don't think his child is a bad person but that his dad's permissiveness is not doing him any favors because it's going to be hard for him to make friends being such a smart A.  Also... if he falls too far behind in school, the best that poor boy will be able to do is flip burgers.. and you KNOW he should be more capable.  He clearly can succeed when he puts his mind to it and the fact that his parents aren't holding his feet to the fire more means that THEY are the ones that are contributing to the fact that he is likely to never reach the potential he should.  It is infinitely more difficult to catch up on years of slacking off than to just keep up as they go along.. but his parents need to MAKE him do it.. because he is unlikely to make that good choice for himself.  His parent's job isn't to be the most popular adult with him.. their job is to raise a productive citizen and right now they BOTH are falling short.

Yes, I definitely recommend counselling for everyone! Counselling will help BOTH of you get on the same page. 

Here's the the thing. You and I have VERY different parenting styles. This is ok because we don't parent together. I can tell you I would probably react the way your DH is doing (hopefully not on schoolwork) so I'm putting a lot of what I'd do in my response - so this is just a guess...  Here goes:

Your DH is fine with you disciplining your girls in the way you do, because they are your girls. But he is not so good with you parenting HIS son the way you do because he does not AGREE with your parenting approach and you have put him in the position of being the soft one because you are strict, and he's making up for always being on that kid. He has almost certainly gone too far the other way - because he really should be tackling the failing grades and not just throwing his hands up. He may or may not be lazy or soft or unskilled. 

Re. your girls...who chews their *ss if they leave a towel on the floor? Is it just you or is it DH, too? Or is he saying "You better get that towel up or you mom's gonna blow a gasket." Is he enforcing your rules for them or is he naturally much harder on them?

Re. L - he is rebelling against you because he sees his dad undermining your parenting approach and let's face it this parenting approach isn't working and it's rubbing him raw. He's reacting to you because he's sees you as the source of all the pain. I get the sense that you like authority and order (hence getting your girls to say Yes, Maam, etc) - but other people don't get the same sense of security in order that you do - they actually find it chafing and constraining. If he has ADHD he may really struggle with order and executive function, he may find tidying his room really overwhelming unless the pain of not doing it is bigger than the pain of doing it. (I am actually like this. I wish I were different, but I find returning things to order overwhelming and ultimately unsatisfying.)

I get that you are doing this out of love - and I believe that - but it isn't working. Your DH will NEVER back you up so long as he doesn't actually agree with what you're doing, but if you have AGREED rules then you have every right to expect him to back you up. You have to negotiate these rules. As you haven't been able to do so on your own, there's zero shame in getting someone to help you with that negotiation. I have to occasionally get people to organise my stuff and my life because this is something I lack skill and aptitude for, yes sometimes it's hard to admit you need help with what seems pretty basic, but we can't all be good at everything. 

I'm pretty soft when it comes to discipline. When it came time to blend families, I had to tighten up some and get stricter where it comes to my son. My partner isn't exactly a disciplinarian, but he's stricter than I am. He had to soften up where it comes to his kids. We have to work constantly to be on the same page, to be fair and appropriate for each child. You are going to have to do this, too. What works with your girls right now isn't working with L, they are different people and different genders. What works with your girls today may not work tomorrow. 

Listen. I get it. I also was put in a "you are responsible but have no authority" position. 

I left. 

You are right in thinking "what am I going to do when he is bigger?". Which, depending on your size, could happen when he is 13 and full of all kinds of attitude and "10ft tall invincible" thoughts. Which, you know what he IS invincable, because your dh AND mil make it that way! (Ugh about the mil, I dealt with that bull crap also!)

I am sure resentment is growing towards your dh because of this and eventually that resentment is going to seriously effect your relationship. Also, you really really need to think about how this is affecting YOUR children. They also will start to resent not only your dh and ss, but also YOU. They have to follow a different set of rules and probably do chores etc while ss has a different set... no, lets call it like it is - he has NO rules. 

You and your children are being treated like 2nd class citizens. Can you seriously live with that long term - do you really want to have your children grow up like that?

You need to sit your 'd'h down and make him understand all this bull crap is NOT ok. If he is unable to wrap his mind around it, I suggest starting to implement an exit strategy. It will not work long term, why waste your time and put yourself AND your daughters through this?

My house, DW has her kids keeping their rooms clean (she helps her son) and asks them to do occasional chores (her daughter takes initiative and does extra chores without anyone asking, while her son plays games to avoid doing what she asked him to do, and DW STILL has him as her pet). All I ask is if these two kids can help out some, show a little pride in our home (it's their home, too, right??), and be respectful. Being that they're teens, their respectfulness is highly limited, but the boy (just turned 15) thinks he's somethin' special. Hate to break it to him, but he's no more special than anyone else around the house, even though DW treats him like he is.

Welcome to the reality of IT'S NOT YOU, IT'S NOT YOUR SKID, IT'S YOUR SPOUSE'S (ineffective) PARENTING. Their lack of appropriate parenting is the problem, even though some of us could benefit from backing off of the kid and going to the spouse to say what's up.

I'm trying to back away from resenting DW's kids, since they're doing what she allows them to do. It's HER PARENTING. "Oh, but you both have different styles." No kidding? Thanks for the session, doc... Problem is more basic. She felt like her dad was a control monster and didn't like it, so she vowed to not be like that - and so her (almost 14 YO) daughter is a conniving, lying, sneaking, sassy brat who knows how to stay in good graces by doing extra chores; her son is the apple of DW's eyes and can do no wrong in her mind, so he gets to do whatever he wants (which makes her daughter jealous to the point she acts out).

I sometimes wonder WTH I'm still doing there. Just call me "Mort" - full name is Mort Gagepayment. Grr, frustrated lately.

Like me, your spouse's parenting is the problem. Get onto him. Ask him if he loves you enough to insist his son respects you. Ask him if he loves his son enough to insist his son learn some self respect and respect of the standard rules of etiquette.