Sign of an inbred guinea pig?

Rodents in the wild don't mate with siblings if they can help it, they'd rather look for an unrelated mate, however if it happens, it is not as genetically problematic as it would be with humans. Humans who inbreed often have physical and mental deficiencies because human DNA, even in unrelated people, has very little variation. In animals like rodents, there is more variation in related groups. I don't think it will cause too many problems, and as long as he is healthy and happy, eating and drinking, able to groom himself, then there should be no reason to think there is anything wrong. I have a female hamster that came to me pregnant by a littermate, and she had 5 healthy, hyper, fat little things that all are doing well in their new homes.