I have never had to use dia...

Our pediatrician is the probiotic guru, she won't even talk to you about anything that might be wrong with your child unless that child has already been on a strict regimen of probiotics, so if you have any questions I might be able to help you because we've been through the wringer a few times.

We like UDO's.  You can get them in Infant or Toddler capsules. My middle child gets yeast infections any time she takes antibiotics, and the probiotics are really helpful AND even if you're not sure that that's the problem, they are certainly NOT harmful, especially if your son has digestions issues to boot.

Another thing you might consider trying is using Tea Tree oil in a solution to wipe his bottom. It acts as a gentle astringent and really seems to help keep things clear. Instead of using disposable wipes you can use old washcloths and make a solution of Dr. Bronner's and Tea Trea oil. It's amazing and it really cleans things up. 

Good luck!