2. Let me make guesses: he is picky at table; he takes a few spoonful, then runs away from table; you give him a lot of snacks at nigth. If that's true, you just have to spend a lot of time to break him into good eating habits. I will check back if you have more to discuss.
3. You may need to take him to a dentist for a check.
4. Pediasure is basically sweetened milk. Don't have to use it if you can.