I don't know of any OTC low fat dog food, but you could get a prescription type of low fat from your vet. You could also cook for him, and it's really not all that difficult and probably less expensive than a commercial food. Boiled chicken, boiled potatoes, a canned veggie like carrots or beets or green beans. This is what I do to supplement The Fuzzbuttz dog food, and of course they love it and it's good for them too.

Hope he feels better soon too!