K.
I know it's so sad to see our babies hurt, but especially their little hinnies :( My son just went through a similar problem except his diarrhea was so bad that his stomach acids burned his skin and he blistered on his little bottom. Now we are caring for his wound much longer than the diarrhea lasted.
For your question though, My sister totally swears by the BRAT diet for diarrhea - her son gets it with EVERY new tooth. BRAT = bananas, rice, applesauce, & toast. I tried mixing brown rice with applesauce for my son, though and he hated it. So what I've been making is his infant oatmeal mixed with applesauce or bananas and that has helped a lot in firming things up down there.
K.