Pig

How do I encourage a 16 year old boy to eat?

He doesn't like the feeling of food inside him, the feeling of eating, but he doesn't think he's fat. He knows he has a problem but he only eats enough to function. He shakes occasionally which I think is due to low blood sugar. He can piggy-back me though so it's not like he's frail. In fact his arms are quite ripped. I have never seen him drink any fluids but he doesn't have dry lips. I don't know what to do. Help please.