What does it mean when guinea pigs jump around in their cage?

He is probably perfectly fine. Guinea pigs have unique ways of showing their delight. One of these include "popcorning", a series of small jumps the guinea pig does when he's happy. However, you should check around the cage and take a look at the daily routine to make absolutely sure nothing is out of the ordinary. If you feel this is a problem, feel free to contact a vet and get their opinion.