infant thrush help 😥

my baby had a severe case of thrush when he was 2 weeks old. He’s almost 4 weeks old now and it came back. I always boil all nipples and pacifiers after every use, so I have no idea what I’m doing wrong. I don’t breastfeed at all. He is formula fed only. Why did it come back after 10 days of treatment? Ugh. Will the doctor just prescribe more nystatin or is there another medication for it?