Agree with you pixie. I believe people wanting reassignment surgery on the NHS have to meet quite strict criteria [though I haven't researched what these are], but aside from that if an adult has the wherewithal to go private I'm sure they would find a willing surgeon somewhere. It's worrying that such irreversible and drastic procedures can be entered into when the circumstances might turn out like your son's.
I don't wish to make light of your family's experiences but I saw a comment the other day from someone who is 'de-transitioning'. They said [something like], 'Twenty odd years ago I'd have just become a goth for a bit'. That's very much a simplification of a very complicated area, but I'm sure you'll understand the point and hope you understand why I added it here. All the best to your son.