This sounds like it might be juvenile acne or cellulitis.
This can sometimes be cleared up using benzoil peroxide wipes or hydrogen peroxide to clean the area followed by neosporin or triple antibiotic ointment on the spots. Try not to pop the pimples as that helps them spread.
You may need oral antibiotics and steroids to resolve this too.
If you feed out of a plastic bowl you may find swapping to a stainless or ceramic bowl that you keep well washed helps. And you may find washing his face after he eats helps as it will keep food out of the skin pores in the chin area.
If your vet cannot help you then you may want to see a veterinary dermatologist
http://www.acvd.org/site/view/55607_FindADermatologistAlphabetical.pml
Hope this helps you!
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