This morning I noticed my 3 year old guinea pig hopping around on just his back left leg, the right was tucked under him. I thought I noticed him favoring his right leg last week but then everything seemed to be normal and he appeared to be walking fine. Doesn't appear to be in pain. He is eating and otherwise seems okay. Is this
Something I should be worried about and see a Vet?

This sounds very much like it has either dislocated its hip, or broken that leg. Best have a vet examine it for the answer.Your choice what you do after that...

You should take him out of his cage and carefully touch his legs that he "is not using" If you hear any type of squel you should take him to the vet. I speak from experience I have 3 guinea pigs and just did my science fair project on guinea pig behavior.. lol. Hope this helps.