The only thing that you can really do for him at home is wash the bald area with peroxide or betadine and apply some neosporin ointment. This will help hasten healing.
For the diarrhea, most vets recommend switching to a bland diet. You can use boiled, chopped chicken or even jarred baby food meats. Just choose a brand with no garlic or onions.
Then see your vet if you don't start to see some improvement.
Let me know if you still have questions.