I hear your desperation, Tired Mommy. Boy, I'd be tired, too. And I'll bet your son is also tired, but doesn't know that there's any other way to feel, since this has been pretty much his whole life.
It might be worthwhile sharing your concerns with his pediatrician and asking what resources are available for children's sleep issues in your area.
Because his problems have been so extreme, I'd also be strongly inclined to seek out alternative approaches.
Chiropractic adjustments might be in order, to help relieve postural problems resulting from tense muscle habits – I speak here as a person who has had severe asthma problems all my life, and the struggle to breathe can cause severe neck, shoulder and chest tension that makes relaxing into sleep pretty difficult.
I've seen yoga recommended for kids, and a good teacher can make this really appealing. Yoga is another helpful path to physical and emotional relaxation.
Finally, I also have trouble sleeping if I've had ANY recent exposures to scented products or most common household cleaners. Since you've been working with his diet, you might also consider bagging up all air fresheners, scented toiletries, fabric softeners and soaps. Try an unscented detergent for dishes and for laundry, use baking soda and vinegar to clean everything else for a month or two and see if this helps.
I sure hope you arrive at a solution. Time may be part of it, since your son has spent most of his life struggling for his very breath at night, and only recently has that problem been relieved. He could very well need every possible assist in getting to a normal sleep pattern, since it sounds like he's never had that before.