Fiderer
Hey, pag the elder had that around that age. He is also expert on the internalising stress front.
He is 18 now. He has primarily recognized that he needs to excercise and eat well. I know that sounds really trite but a huge crash around Christmas just confirmed it. He is now running regularly and signed up for rugby at uni and is doing much better.
Obviously first term at uni was a big stress so we were not surprised tbh.
The other thing that helped when he was smaller was deep pressure massage. No poncy light rubs - a good tough massage and then (weird alert) putting blankets over him and lying on him.
You can point and laugh if you like 
Interestingly pagboy has just started asking for massages too, just as his verbal tics have increased and he has started to stammer -a sure sign of stress in him. And he sometimes intuitively knows what he needs iyswim