Check his nose. Is it runny or blocked? If so it could be back-drip, mucus from the nose dripping back down the throat when he's sleeping. He may be having a cold or allergies, and should get it treated soonish to avoid a secondary infection like ear infection.
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are you sleeping in air con? if yes, better not to sleep in Aircon. avoid all cold drinks. can apply some yuyee oil to keep baby warm too.
In TCM, they would call it the cold cough which mean his inner health is more to the cold side. Avoid cold drinks and all for now.