Both my cats have done this at different times. I'd say if it happens that often and it makes you nervous get him checked out by your vet. If it's nothing - great, if it's not - you catch it early. With my kitties, it ended up being hairball related and there are special foods that can help with that. One of my kitties is on prescription food now for struvite crystals, but the other one gets Natural Choice Indoor as well (like someone else suggested). It seems to work pretty well. But I would take him to get checked out just to be sure. It never hurts to be a little paranoid when it comes to this stuff.