Ginger, your DH doesn't sound like an abusive man to me: he sounds like an exasperated parent of a pre-teen who isn't handling it well.
Since you say they were close, your DH is probably feeling upset and threatened and even a bit bereaved by the changes. When you've spent the younger childhood years giving and nurturing and hoping, it can be awful when you suddenly realise you're not in control any more, and they may not turn out to be the person you'd hoped. You may feel guilt that you did something wrong, or not enough. You may feel resentment that you put in all that effort, and it seems to have come to nothing. You may feel fear that they won't grow into a 'good' adult. You may feel terror that they'll 'go bad'.
Those feelings are perfectly natural, but as a parent you can't get stuck in them. You have to recognise them as your issues, and not make them someone else's problem. Particularly, you have to realise you can't dump them on your child, and if you are inclined to, you have to work out how not to do it...
Of course your DH may not know what he feels: it took me at least a couple of years to work it out myself. It sounds like he knows he's behaving badly, but not why, or how to stop. I had friends and counseling to help me; he would probably benefit from the same. :) He must do that work - I do not think it is optional; it is the 'adult' thing to do, and if he doesn't, he himself will stay a child or teenager, emotionally.
All the specific arguments and niggles - the school work, the table manners, the music practice - are neither here nor there: they can be dealt with, or not, practically. The reason it feels like bullying is because it isn't rational; it isn't about that thing, it's about the underpinning fears. And it 'belongs' to your DH, not your DS. If it wasn't broccoli, it would be the way he buttoned his shirt, iyswim. Your DH needs to separate out the things that need tackling in your son's behaviour, from the things that need tackling inside his own head...
I was also the compliant and hard-working child of a very driven and aspirational parent. When you yourself have reacted to parental demands by always striving to meet or exceed them, it can be at first unfathomable to find your own child doesn't do the same.
In the end, I recognised that my son's independence and determination to forge his own path, while uncomfortable for me, were healthy for him and just as likely to lead him to success in the end. In fact, though my DS appeared to totally reject all I could offer him (education, learning, work ethic, kindness, all my moral values...) and that was painful and difficult in many different ways, he is now, at 18, 'coming back' round to them - but his beliefs and actions are now his, rather than simply 'inherited', and I have hope that he will be the better man for it.
Maybe that will be some comfort to your DH, and you. :)