With regard to the exams, I know what you mean, but my advice would be to just back off and leave him to it. That is what I have always done with mine. You can't do the work for him, and you can't make him want to do it either. It really isn't worth the hassle.
They are old enough to know at 15/16 what the consequences of not working for them are, and if they need to learn by experience then so be it.
My experiences are as follows:
My DD1 (now 20) was fine, very self-motivating, academic and bright. Eager to learn, and learned easily. She did it all herself with no input from me unless she asked for it (which was rare). She got top GCSE and A Level grades and is currently doing well at a Russell Group Uni.
DD2 is 16 now and supposedly doing GCSEs over the next few weeks. She is not academic at all and her predicted grades reflect this. She is almost frightened, I think, of academic stuff and seems to have little inclination to study. She has struggled academically throughout school, but is more practical than anything else. Impossible to know how to motivate her as the more you try the more she digs her heels in. Best to back off and let her make her own mistakes, being there for her when necessary and to help her find apprenticeships, which I think are more suitable for her.
DD3 will be 13 this summer and is just finishing year 8, so too early yet for public exams. She is very like DD1 though, and wants to get to uni. Academically bright, sporty and very self-motivating. Wants to learn and seems to do it very easily. She won't need much input from me either, just when/if she asks.
My problem is that the only one of mine who really needs help is the only one who will never accept it. That is DD2, and I do feel for her, but there seems little that I can actually do.
None of my children has ever physically assaulted either of us though. It would be a real game-changer if they did and I might not rule out involving the police. What you describe, OP, could possibly be termed domestic violence, albeit from your teenage son. I would make clear that this was the first time AND it would be the LAST TIME. Hard though it will be, perhaps police having a word with him might make him think about his behaviour a little more, as his father doesn't seem motivated to tackle him about it.
Back off from him with regard to studying. Leave him to make his own balls-ups with regard to his revision and seeking work/further education. Be there to help if and when he finally realises he has screwed up (lets hope that he doesn't though), but make it clear that there is certain behaviour you will not tolerate, and what the consequences will be, whether your husband is on board or not.
Good luck. 
It is sooo hard, isn't it. Teenagers can be loving, but some can also be shits/spawn of satan, call it what you will. Most emerge on the other side as good and lovely people, but getting there can be such a challenge.