I have many questions about what kind of diet your son is on, so without any knowledge of diet or lifestyle, my first suggestions are: stay away from the obvious causes of yeast infections, such as antibiotics and sugar. There are several different ways you can begin to introduce friendly flora back into his system, but I would need more information.

What type of diet is he on?
Has this been a problem since birth?
Was he nursed?
Has he been on antibiotics?
What types of medication are you referring to in your post?