Yeah, I would say first a vet visit because if it is stones or impaction, that could be why he isn't being active. Second, I think you'll be surprise by how much more active he will get with a cagemate. Also, toys and hidey houses are fantastic for getting them more active during floor time. Here is my set up, and my
cavies absolutly love it.
My living room is hardwood floor with a carpet in the middle, which I don't remember how big it really is because the tag is faded, but its roughly the size of a 4x7 cc cage. Which works to our advantage because the piggies hate being ont he hardwood and will usually stay on the carpet. We have 3 piggies and 4 hidey houses. We put a hidey house in each corner, facing the middle, although they don't stay that way, and a little foot stool in the middle. They love to run and chase each other from one house, under the stool, to another. And Heidi really seems to enjoy just running circles around the stool. For extra protection, we some newspaper under each hidey house and the stool just incase they stop to use the potty, but typically what happens is they will go over to their cage and start chew on it when they need to potty, they'll do their business they start wheeking to let us know they are ready to come back out. usually happens once per pig, per hour they are out of the cage.