S.C.
Go to the local drug store and get Gentian Violet (over the counter). It is a purple dye, that kills the yeast/thrush. You can apply it once a day to his mouth (with a Q-tip), and to his thumb (no need to try to stop his thumbsucking). It works WAAAAYYYY better than Nystatin. It is an old remedy, that docs have forgotten about. Typically the thrush is gone in 3 days.
I also put Vaseline around the lips prior to using the Gentian Violet, to keep the staining around the mouth to a minimum, wiping the drool off as needed. Your little one will likely pucker up as if responding to alum (a pickling spice).