Maria, what you're feeling is frustration from caring more than your DH does. 

What is happening for you is you are witnessing a man half-*ss parent his kid. For whatever reason your DH has found himself to believe how he is parenting his son , his way is sufficient. You know, the kid wouldn't be even getting the meaningless 'lectures' if it weren't for you giving your Dh the stink eye . 

Of course you're absolutely correct that a kid's education along with homework (and the importance of the process) needs to take a priority. Your Dh may actually also believe this.... but he doesn't have the 'want-to' to make it happen. He doesn't want to be the 'mean parent' who denies his kid video game time because the kid might get angry. Or the kid might decide kid doesn't want to come back to Dad's. A lot of these parents who are parenting (or not parenting) exactly as your Dh is are doing it because they feel guilty or they feel afraid or both. 

Your Dh tossing money at the kid and letting the kid do and have whatever kid wants is not uncommon and you will make yourself old and unhealthy real fast if you keep pushing against what your DH is doing. Why? Because you are caring more about how this kid is being parented and raised than his own mother and father does. In a perfect world, that might actually be a good thing, right? I mean at least somebody gives a d*mn about what's happening with this kid. But in stepfamilies that is usually not how it works out. 

You rapidly become the 'evil SM' in your SS's eyes and the nagging busybody who has no clue how to 'parent' in your Dh's eyes. I bet you've already heard your Dh announce ' you just hate my kid' . 

Welcome to Steptalk. Lots of ladies here in the same boat you are. I'm sure some of the members will give you advice on disengaging and how to do so. It works for some. No, it doesn't help the kid, but it does sometimes save your sanity over what's going on in your household. 

Most kids are going to take the path of least resistance that gives them the biggest reward. Lying about homework with your DH results in no discipline (stern talking-to's aren't discipline) and ample game time. So, SS lies.

I would disengage but be very clear with your DH about your expectations:

"DH, it is clear to me that you and I don't see eye to eye on the homework issue. From now on, I won't be part of it. Do not make me the gatekeeper of privileges. I will keep my nose out of your business with your son around this. However, know that I will NOT live with an adult child who has not launched, so if you foresee our marriage lasting long-term, you need to prepare your son to be a productive adult."

That sentiment will anger your DH, but I personally think it is better to set him straight about your expectations now than waste 6 more years wondering what you could have done differently. Ultimately, you can't do anything other than disengage or leave, so state your peace, don't argue about it ("DH, there is nothing to discuss about this; I have said my peace"), and let him decide how he will parent.

This was one of my biggest irritations before I disengaged. 

I thought I was 'helping' when I got involved with homework but realised I was just the bad guy as I was the only one with any expectations of SD.  My SO & her BM just saw pink roses whilst I saw hours of staring at a blank page and listened to lie after lie, my personal favourite was SD telling SO over the phone for a week she'd seen her private Maths tutor, she even told him the contents of the lesson only to be caught out when BM mentioned something they'd done on the same night, when SO queried it BM threw her own DD under the bus with the words "You've dropped yourself in it now".  SD had lied with so much conviction, BM knew she'd lied as she's there when SD speaks to my SO and then for BM to push it back on her DD, we were gobsmacked. 

But guess what, my SO just gave SD a stern talking to and a day ban on her phone, did it work?  Nope! 

This was years ago and still I hear lies about homework, it's flagged on every school report (SO tells me) and mentioned at every parents evening (SO tells me).   I got to the stage where I decided if it wasn't important to her own parents why should it be important to me.  I would step back now before you become the target of the bad feelings/mouthing from all sides.

Me too!  I recently posted a blog that discussed this very topic.  My SD15 was flunking 3 classes as of last week.  She got a paper turned in.....not a re-take, but a re-re-take, so at least one grade has been pulled up, not sure about the others.  Don't even get me STARTED on the school system that allows such nonsense.  But, as the others have so eloquently said....you can't care more than the parents.  I have stepped back from asking/nagging about homework being done, but I do ask the occasional POINTED question about grades and missed assignments....just out of concern and curiousity of course....and just say "hmmm" when I hear about how far behind and how many classes she is flunking.  I have also made it VERY clear, and FDH has too......no loser, failure to launch skids living in our house after high school!