I agree with the others, he isn't responsible for your happiness but you are both responsible for the health of your relationship.
I'm going through a similar, but lesser, thing in reverse atm. My DD has gone to senior school and no longer benefit enough from me being a SAHM for me to forego the fulfilment of a career.
I'm lucky, my marriage is good but I've realised that I need to change the things I can (I'm currently studying so I can get a career, rather than a job). My DH has taken it hard that I'm unhappy as he sees it as his job to make me happy and sees any unhappiness as failure on his part, rather than just down to things I have to be responsible for and that he can't fix. He's very down that I'm not 100% happy like I used to be but being 100% in supporting me in making the changes I need to in order to fix what I need to fix.
So in your case, I think you've got 2 things here. You need to work out what you want to do in the work place and then set your mind to achieving it. I never thought I'd be in full time education aged 41 but I've got a good 27+ years ahead of me and more than enough time to achieve in the workplace if that's what I want. You can too! It's not about the age you are now it's about the time you have left. You either accept your years until retirement aren't going to be fulfilling or you do whatever you can to make sure they are.
Then there's your relationship. What's needs to change for you to be happy, and is your DH willing to work towards change? Is it salvageable?
Personally I think if it isn't and I was in your position, if it were possible to continue to live in status quo without things getting worse I'd stay and get trained/educated in whatever path you want to go in and then LTB when you're in a position to push forwards on your career. If it's all too much to stay and you couldn't focus enough on you then I'd crack on with the training now anyway and leave when your son does. Many relationships end when a child leaves home, my own parents split up soon after I left home.
Good luck with whatever you decide to do but remember you were a high flier, and although you can't immediately go back to that (and I know how hard that is) you have the same personal skills you used to plus a whole lot more now so just need to build up the technical stuff. You can do that for yourself and will feel so much more self fulfilled when you do 