I agree with Teagz that you'd need to talk to his parents unless your his carer. But when he is with you (sounds like you with him a bit) there's nothing stopping you making him nice yummy healthy food! There are loads of healthy recipes kids like, as long as they don't realise it's healthy, lol. And you can always chat to him about the benefits of healthy eating.

On the flip side, does he exercise much? Could you arrange taking him out for a walk or bike ride each afternoon or something?