I can't answer your question about what is causing your son's problem. What I can tell you, from one mother to another, is that if you're not happy with the care your son is receiving, you absolutely have the right to seek that care elsewhere.
What would I do if I were as worried as you? I'd find another physician and get a second opinion. It would at least serve the purpose of easing my mind that my child was getting the best care available, as well as the opinion of two qualified physicians.
Good luck to you.
Kim C, RN