L.L.
Maybe this has already been explored, so sorry if this is a repeat, but maybe he has swallowing issues? I know a couple of friends whose kids went through feeding therapy. They saw a speech therapist for this. Perhaps this is something worth exploring? It sounds like you've been working with your ped, so hopefully he or she could offer some insight?
Good luck. I can see how this would worry a mom to pieces. I hope he starts eating soon.