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If he is active and healthy...you can give 1/2 tsp of Immodium and repeat it in 12 hours. Pull up his normal food and give rice and chickenbroth for a day or two. Make sure he doesn't have access to cat food/poop or other "treats" that could be irritating his GI...like rawhides.
If he becomes lethargic or vomiting...you need to seek out a vet.