My son is 3 - he is taking medication for allergies. he recently started snoring and has foamy saliva when he is asleep, what could be the cause?

Monitor the setting: A 3 yo who snores has tonsil & adenoid growth that is prominent & interfering some with the airway. If he has breathing pauses lasting more than 20 seconds during sleep, he is progressing toward childhood sleep apnea. This usually worsens for a few years since tonsils/adenoids grow faster than the kid until they hit 8 or so. If this occurs have him evaluated by an ENT. Tonsillectomy is curative.