I would definitely get your baby tested for food allergies ASAP if you haven't yet. His symptoms sound a lot like my son's, who was allergic to soy (as well as dairy). My son had all kinds of gi problems, occasional bloody stools, excessive spitting up, not gaining weight well; plus he had congestion, runny nose, wheezing, and just general ill health. Fortunately, he was diagnosed at 2 months, and he was like a different baby once we changed his diet. It is a lot easier to get the skin tests done at an allergist's office than to keep trying different foods on your own. With Celiac being in the family, he should definitely be tested for that as well! I find it is helpful to TELL the pediatrician specifically what you want your child tested for, or what specialist you would like for him to see. Never assume the doctor knows more than you, and do not let him assume it either. If he is uncooperative, find a new pediatrician! It is easy for the physician to see the child as just another patient to see that day, rather than the precious child that he is to you.
I signed up for this forum specifically to respond to your message. I hope your son gets a diagnosis soon, and gets better.