I don't have any medical advice, though it sounds like you're doing all the right things (with the follow-up, etc). One suggestion I have regarding the medical care/diagnosis is to take him to your closest Children's Hospital, even if its far away, because the will have tons of expertise on babies/children. And ask if your pediatrician can get you into see a pediatric pulmonologist, not just a regular pulmonologist.... If your ped. can't, have him/her contact the Children's Hospital to see if s/he can get your a referral to one there....
My only other thought is for you to hold your baby as much as possible in an upright position between your breasts, with the head turned toward your heart. This is used with premies a lot (it's called "Kangaroo Care") and has a significant positive impact on their health, including heart and respiration. Get a baby carrier and strap the baby on, something like a Wrapsody wrap, or a mai tei carrier etc. (you can see various carriers at www.attachedtobaby.com, or tons of other websites; just google "baby carrier"). My thought is that it sure can't hurt to use the carrier and keep your baby close, and I noticed with my (full term, normal, healthy) baby that he loved, loved, loved being carried (what baby doesn't? :), and I was able to carry him "hands-free". :)
I'll be keeping you & your little on in my thoughts...