Time for a sleep study!
Although.... a thing to consider... many kids have a very active gag reflex. WhenEVER my son had a sinus infection or postnasal drip at night until he was about 5... he'd throw up at night. At age 9 it's 50/50 on whether he'll gag or not. The mucus would roll down the back of his throat in his sleep, hit his gag reflex, and bang. Time for new jammies, sheets, the works.
At the time, they still had infant pseudoephedrineHCl drops on the market (manufacturers have pulled them because once they went behind the counter -no Rx needed, but a drivers license and pharm hours only-, and the referral to see a doctor before using switched from 6mo to 2 years, they didn't sell well. Then, when parents started OD'ing their kids on the meds that didn't work that were on the shelves. Well. It's a pain. There is no longer infant and toddler dosing available in this country. Although it's available in every other 1st world country.) Pseudoephedrine is the ONLY effective decongestant for sinuses. Our ped had us cut the adult tabs in half, crush, and mix with Karo syrup or chocolate syrup until our son was old enough for 1/2 the adult dose (aka 1 pill). NOW... you'd need to speak with your ped to find out the correct dose per weight. Those pills are pure magic in our house. Otherwise, I'd be cleaning up vomit all the time. But 2 or 3 nights of decongestant (NOT antihistamines) and his sinuses dry out and he's good for a few months.
Since your son is prone to ear infections, he's HIGHLY probable to be prone to sinus infections as well.
But there could be dozens of problems. From reflux to a neurological disorder (many kids have mild seizures while they sleep which *can* cause voiding in one direction or the other... or apnea which can be scary enough that when they wake they gag.). Hence the sleep study.