Okay, yeah he's bloated. Sorry, I don't mean to doubt you or anything!
Have you noticed little orangy/brown curly solid looking things on the gravel in the tank or anywhere at all? Those would be his poops, you don't necessarily have to catch him in the act of pooing but just to see the poo is good.
Otherwise, there's something you can get from your pharmacy or wal-mart. It's called Epsom Salt, you won't find it at a pet store. You want to look for 100% Magnesium Sulfate, no additives, no scent and no dye, 100% pure. It's a natural laxative used to help reduce swelling and it helps sometimes to get a fish's intestines moving to clean him out.
You'll want to use 1 tsp per gallon, so say it's a 5 gallon tank, you'll use 5 teaspoons. ALWAYS dissolve the salt before putting it in the tank, when salt dissolves it actually creates heat which can burn your fish if you don't dissolve first. So I use a little plastic cup and some tank water and generally dissolve 1 teaspoon at a time since it's easier. I swirl my cup around, you can stir or do whatever to get it to start to melt. Then using a teaspoon spoon 1-3 teaspoons into the tank every 5-10 minutes so he can get used to it over a little while.
You can keep the salt in until your next water change. If he hasn't pooped or slimmed down in 3 day's then we can either up the dosage or you can try to find some Daphnia, Petsmart sells some generally in a small container with both freeze-dried bloodworms and freeze-dried brine shrimp. Don't give your boy freeze-dried bloodworms at all, if he's got issues digesting things then those will only make it worse. But Daphnia is a natural laxative and much easier for them to digest than using salt.