Personally, before trying those, I'd do some more natural things first. A yeast rash signals a flora imbalance in his gut: get him some baby probiotics and give them to him as well as sprinkling some on the rash. You can also apply plain yogurt (Stonyfield Farms has the most live cultures) directly on the rash, and add some apple cider vinegar to his bathwater.
I've had luck too with a few drops tea tree oil (a strong antifungal) added to some olive oil and applied to the rash (test it on yourself first--it can sting if it's too strong).
These things work really well, btw.