Certainly sounds like it could be. You have a couple of options - you can give him some nystatin oral drops (nilstat) which will kill the thrush, and as suggested, if you are breastfeeding, some cream for your nipples as well. You could also put him on a course of probiotics as well - some of the major health brands do probiotic powders that cater especially to children, or you could use some yoghurt with live cultures. These will help restore the balance of good bacteria that the antibiotics disrupt and help his body fight off the thrush naturally.