Ay, heat rash talaga yan kaya mainit-init! A few of us moms, ginagamit namin ang cornstarch or talc powder after bath, and make sure ang likod is completely dry bago magsuot ng damit, sometimes ang sweat lang talaga ang culprit pag malambot ang fabrics. Try changing clothes more often or use loose cotton shirts, at siguro naman better yung airflow sa likod niya, lalo na ngayon that it's getting warmer. If it doesn't improve in a few days or kung umuubo pa siya or feels hot, check with your pedia na lang para sure.