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It's best to feed exactly what the last owner fed for a few weeks til he gets adjusted. Then if it isn't a good brand, you could go to something like Canidae or Natural Balance if you want to continue with kibble.
If you don't know what they were feeding, I'd either try the new food of choice or boil some white meat chicken (no fat, skin or seasonings) and mix it with white rice for a few days because that should set.