A.M.
My pediatrician said that kids get diarrhea or loose stools during teething because of all the extra saliva they're producing. If they swallow enough, it can slightly alter the pH in their gut and cause changes in BMs. If he doesn't have any other symptoms, teething is probably the cause. Just make sure he stays hydrated and monitor him for other symptoms. I wouldn't take him to the doctor unless the diarrhea persists for several days, he develops other symptoms (persistent fever) or he gets dehydrated. (ADD) It also could be a virus. There's nothing the doctor can do for this, so I'd offer the same advice as i did for teething. But that's just me...I try not to go the the pediatrician for every little thing. you know your son, and if you feel like he needs to be seen by a doctor, then follow your gut and take him in (make an appointment; this isn't an urgent care situation).
If keeping his bottom dry and using regular diaper paste isn't working for his rash, it might be a yeast. This is the really red, kind of raised rash with clear borders. For that, I use generic athletes foot cream ( clotrimazole). Apply it 3 times a day and put the regular diaper paste over it. I usually see improvement within 24 hours, but you should continue for at least 3 days to make sure it's completely cured.
ADD: diarrhea from a virus can last well over a week, so don't be overly concerned if this continues for a while. It takes the GI tract some time to heal from the inflammation, and then the pH and healthy 'bugs' in the gut need time to readjust.