M.R.
Get a second opinion, or, if you can with out a referal, go to a speech and languge pathologist or an occupational therapist who works with kids with oral motor and feeding disorders, they can probably help you.
If he is not loosing weight, the pediatricain may blow it off, but at some point soon, he needs to start using the mucles he uses to chew, and he needs to chew more difficult things because he needs these muscles for speech development, and you don't want to have this cause a speech delay too.
M.