My dd had obstructive sleep apnea. She had a cleft palate as well and a small jaw (sometimes known as Pierre Robin Sequence). She also has a small nose, a smallish airway and a deviated septum. The OSA was complicated because of the cleft palate because, like your son, we weren't able to remove her adenoids. She did have her tonsils removed, which unfortunately, didn't seem to have an effect. The sleep study after her tonsils were removed was actually worse than her initial study. We also tried the nasal spray - we didn't have much success with it unfortunately. Thankfully, my dd's OSA wasn't bad enough that she was in dangerous territory - so they decided to not treat it and monitor her every 6 months a sleep study. Other options for treatment were to go ahead with removing her adenoids (and risk her speech), move her jaw forward, making her airway slightly bigger or CPAP. Because her sleep apnea was mild to moderate, the doctor felt the adenoid removal wasn't worth it as besides affecting her speech it likely wouldn't have proven very successful either since there were other things that were causing the sleep apnea as well....it wasn't just large adenoids, rather a combonation of things. The jaw surgery, again likely by itself wouldn't have proven to be worth it given that there were other things involved, plus it was way too invasive a procedure for mild-moderate SA. And because of her age (4 at the time) and the mildness of the SA CPAP would have proven way too difficult for her to tolerate and likely wouldn't have been successful. Thankfully, with time and growth she has gotten much better. She now is within normal ranges while sleeping and while she still snores (quite loudly at times) she isn't having the gasping for air and the stopping of breathing that she used to. So we've been lucky.
For you, if the nasal spray and adenoid removal aren't options and the sleep apnea is severe enough where your son is in a danger zone (for heart attack and/or stroke) or if his daily activities are limited because he's tired, then CPAP is an option. It's difficult for kids to tolerate but some kids certainly do it...it takes a while but it might be worth a shot.
Good luck - and if you have any questions ask away!