mothercat, you have my sympathy. I long ago realised that two people hearing the same words or experiencing the same event can interprete it entirely differently.
You describe him as the 'golden child' and when someone is clever and successful and academic and social success has come easily, it is often a real shock when reality eventually arrives and they suffer some set back. Since life has always been such a smooth ride they expect it as a right and when they do not get it, they look for someone to blame and as his parents, you are first in the firing line. Because you, quite rightly, refused to sign up to a financial obligation you could not afford, you were an easy target and now he automatically blames you when he doesn't get his own way all the time.
I am not sure there is much you can do about it. Just accept that is the way he is, try and step back and do not take his criticisms too personally.
Like others I thnk Smileless has given some very good advice
If you think you could get away with it, try a little banter. If he does something silly,nothing to do with you, like break a plate, immediately apologise for it, and if he says what is to do with you just comment, 'well a mother's place is usually in the wrong'
Only you know whether the above is a good idea or a very bad idea.
You have three other children, who are not as difficult as this child, just accept that that is what he is like, ignore his jibes and follow the very good advice