Piggies and cardboard?

My piggie is 5 months old, and as I've been gathering the materials for a C&C cage I set him up a fake one made out of cardboard boxes. This seems to have gotten him out of his pet store cage, though he still prefers to sleep and eat in the cage, and gets him running. My question is that he has developed a taste for eating the cardboard, is this bad? Always the clean unprinted areas and he nibbles at any corner or side he can get at. He gets plenty of hay, pellets, and fresh veggies and fruit everyday, so I don't think it's a lack of nurtients. I'm just worried about this affecting his tummy and the rest of his health. Thanks.