Does he have any other medical problems which may cause it? I would quit the drinking part and get him to eat, he has to at some stage. There are appetite boosters out on the market, have you asked your doctor about them? My daughter was taking one which was an iron supplement too, I took it as a child and it helped us, helped my nephew and hubby's nephew too. Think of that as an option, may do a world of good.
It sounds like you are giving him Ovaltine, though. Did you try *really* giving him nothing to drink?