Advice

Hello I'm Shelley. I've just joined. I was wondering if anyone had advice on autistic teenagers? My son is 15 and received his diagnosis two weeks ago but obviously I always knew. I'm having a really difficult time over his weight. He's become obsessed with his body. His foods are carefully selected and weighed. He knows the calories of everything etc. He's so upset at the thought of putting any weight on and has lost quite a bit over the last few months. He does eat and reassures me he knows what he's doing and it's not an eating disorder. He just really needs to feel comfortable in his own skin and to do that he says he needs to be a certain weight. I  just don't feel qualified in how to deal with him. Sorry for the long post.