It really depends on which teeth are missing. I am assuming that these are lost baby teeth, and his permanent teeth have not erupted? If this is not the case, he will eventually need to have these teeth replaced, but I'm sure you've been over this with his dentist. Until then, just keep encouraging him to eat, and he will. He's probably having trouble dealing with the missing teeth, but will adapt with time.
Seeing a dentist is advised.