Son Dealing With PTSD

My son is 28 now, and he experienced sexual abuse when he was only four years old, inflicted by his cousin who was 13 at the time. I didn't learn about it until my son was 13 myself, back when I was a single parent. At that point, I had no idea how to handle it.
Now, because of what happened and me not knowing how to deal with it properly then, people are saying it's somehow my fault that he still lives at home and doesn't contribute financially, suffering from PTSD as a result. He has seen a private psychiatrist and done lots of therapy, but he says he just can't function.
I'm really unsure about what to do nxt because his GP isn't very helpful. I think the NHS might have given him all they could over the years, but private treatment is incrediby expensive at £350 an hour.
All this has made me feel so guilty and helpless. On top of everything else, my mom who has PD lives with us too, in the UK.