In my kid's case it was something in him that clicked.

My youngest was the kid who caused his teacher to gray. Capable, but not interested in school. He'd do just enough to get by. He'd say: it doesn't matter if you get an an A or a C. you go to the next grade anyway. Then as a high school junior, he decided he wanted a certain job as an adult and wanted a special college program. He went from scraping by to all A's. He's my only child with a Masters. He actually said he hated school, but would tolerate it so he could get the job he wanted.

I never sat with him while he did homework. At 13, I made sure he had time to do it and made him responsible for it. He did enough to not have detention as I wouldn't pick him up and he didn't like the 1 1/2 mile walk home. 

I always hated school and have never been the student who needs to get straight A's so it might not happen with your SS. My parents stopped helping me with homework in like 6th grade unless I asked for help and I was held accountable based on the grades I got.

I agree with Tried Out that as long as his parents force it and help him he probably won't take responsibility for his own work.

Some never do. My SS20 was like your SS.  BM held his hand with homework, DH didn't. Eventually he was alienated and BM just gave up on homework and he was not held accountable for it at all; in fact, she made excuses to the school for him.  He graduated because his special school program dumbed it down and dragged him through it.

My DH also hated school. He graduated, but never cared about grades. He went to college in his 40s and did great.

I, on the other hand, cared about school in First Grade.

I think you are waiting for your SS to suddenly become "normal" and I'll tell you, with parents like that, it's not going to happen. He's going to be like my SS - almost 21, living with BM, doing absolutely nothing with his life.

The sometimes never answer you are getting is basically the bottom line.  My YSD always was driven to excel in school.. she was naturaly smart and it was a way she got positive attention.  She thrived on being on deans list etc.. she did not feel any peer pressure that girls shouldn't be smart etc..

Now OSD?  she was also an excellent student until middle school.. when she noticed boys and didn't want to be a smarty pants... so she just stopped trying.  It didn't help that my MIL would make excuses like "I am stupid so it's not her fault she isn't doing well in school".. I would get so mad and tell my MIL that she (mil) was a lot smarter than she gave herself credit for.. and that OSD certainly was capable because she had just been doing All A work just a few years before and that it wasn't intelligence that was holding her back.. she was being lazy and didn't care about it... and people were letting her off the hook.

I don't hold out any hope for a kid that hates learning at 13.. that is not a time when it gets better.  

Now, his dad could try to get him motivated in other way... does he like material things? well son.. you want nice things? you will need to do better than dig ditches or flip burgers to get those things. Also, does dad try to work learning and a thirst for knowlege into his life?  Our family trips mostly had some educational components.. we didn't go to Disney.. we went to the Smithsonian museums.  We went hiking on battlefields to learn about history... in an interactive way.    Does your husband model the 'never too old to learn" mindset.  Does he read.. does he have interests that require research?  Does your SS have any interests that could be worked into a more educational format for him.

I get frustrated when people rely on schools to teach their kids.  I feel like schools have turned into babysitters... teaching to the "lowest common denominator" in the classroom.  They teach to the kids' levels that are least capable.  The smartest kids are skimmed off to go to advance classes.. the rest of the kids.. capable and not are lumped together.. the more capable bored to death and learning very little because that is all the less capable students can do.  They lose interest.. think it's boring and a waste of time.  My parents supplemented our education in a lot of ways..   The bottom line is that school teachers do the best they can with the rules and raw materials they have.. but they can't work magic.

Smart and capable kids who are capable of performing at a high level academically and don't are infuriating.

This was my Skid for 9th and 10th grade.  It drove his mom and I nuckin futz.  The straw that got him sent to Military School was when he plagurized the final draft of a research paper that he had gotten 100% on the rough draft for.  Instead of just turning in the rough draft as his final paper he dowloaded a paper off of the internet. He was so lazy about it he didn't even delete the hyper links.  He just printed it up and turned it in.  When we climbed his ass for that stupidity he floored us with "Mom and Dad, I know I can do it, you know I can do it, my teachers know I can do it... so why do I have to?  Doing all of that work for something I already know is a waste of my time."

So... off to Military School he went.  He had no choice but to do the work or his world was one of abject misery.  It worked wonders. He was a straight A honor student, lost 40Lbs, and was a star his Jr. year of HS.  

Then.. the Spermidiot got involved and it all went to shit. But.. that is another story.