Hi nevergiveup and welcome to the forum
I'm not sure I can help but your post tugged at my heart strings so I just wanted to say something to you. I'm really sorry to hear this, it must be very tough indeed. He's a lucky young man to have a mum who is clearly trying to help, he must be really struggling internally with how to deal with this and as a parent it must be heart-wrenching.
I do see some of myself in what your son is going through albeit 30 years or so ago, mainly in the withdrawing from life, although perhaps not quite so extreme, but I did avoid a lot of social interaction and even changed schools last minute which was very difficult for me.
Some people seem to shut down completely when asked about their mental wellbeing, my Dad is like that and I don't fully understand it but I'm sure it's a coping mechanism and a way of controlling something when everything else seems out of control.
He obviously needs some professional help and support so I'm sure Ashley or one of the team will point you in the right direction.