Ang baby mo sounds like heat rash (prickly heat), especially pag mainit at maraming pawis, super common sa 9 month olds, lalo na dito sa Pilipinas! Best thing is to keep him cool, dry, at lightweight clothes lang, pero if the redness doesn't improve in a few days or if may signs of infection (like fluid or bumps), better to check with your pedia just to be sure it's nothing else na kailangan ng treatment.