Thank you for your reply.
He's been underweight for about a year - he's always been tall & skinny (but always ate loads), runs in the family, but when his ocd/anxiety really took off he lost weight due to it all.
He's not anorexic/bulimic (not intentional weight loss, no body image issues, except he knows he's too thin).
He likes food and would like to eat more but has no appetite due to always feeling anxious.
He over exercises to cope with the anxiety but is clearly burning far more calories than he consumes. Worries about feeling full and then not being able to exercise properly.
Eats lots of butter, peanut butter, cream, chocolate, fruit, veg, - no actual fear foods but has really cut down on carbs, cakes, biscuits, doesn't eat crisps etc.
Is at school and very bright - on track to do well but we shall see.
He's just started anti-depressants, low dose & will increase, and has been having therapy.
But it's the low weight I'm really worried about and it looks like no one is going to help him. Bloods from gp have already shown very low hormone levels.
I've been fighting for support for him for so long - everything we've accessed so far has been private/health insurance. Which we are exceptionally lucky to have had.
He will never put on the required weight without support. But he feels fine, exercises without trouble and has been functioning like this for a year. So I'm at a loss.