In addition to the very good advice you've already gotten, you might look into cranio-sacral massage therapy. This is an extremely gentle therapeutic massage technique that might be able to ease the constriction in his chest and gut. You would sit with your son the whole time. He can play with toys while the therapist works on him. It looks so gentle, it is hard to believe it is doing anything, but I've seen dramatic results in my daughter and felt them in my own body. www.kenpiercy.com is a great guy - he works with a lot of kids.
I second the suggestions of both chiropractic care and food allergy testing. "Regular" doctors may roll their eyes and call you crazy, but he's your son and you are responsible for his care. Certainly also seek out a pediatric GI, just remember that specialists see what they are trained to treat. You have to trust your gut if you feel (after doing some research of your own, and making sure they took the time to explain everything to you so you could understand why they want to do whatever they propose and what they hope to gain from it) like what they want to do is too invasive or makes no sense. I use both traditional and alternative medicine, blending them based on my own research and instincts. I rarely tell the MD/PhD allergist that the chiropractor seems to be helping a lot...sparing myself the eye rolling...but it works for me and my kids. Particularly for chronic conditions, I prefer to use the alternative stuff. For acute infection or injury I'll go to the regular doctor. I carry an epi-pen for my son's peanut allergy, but he sees the chiropractor regularly to improve his respiratory symptoms (for which the MD/PhD allergist wanted him to take prevacid, allergy medicine and do breathing treatments). I tried the medicine way for about 6 weeks, but it wasn't doing what the doctor hoped it would do, so I stopped it and tried something else. Homeopathy, chiropractic, massage etc, can be really beneficial in conjunction with traditional medical treatment. You don't have to be a fanatic or renounce all medicine to incorporate some alternative stuff into your life.
Good luck,
S.
SAHM of 3