It sounds like he needs more time to find his voice, it can take a month or more before they get really comfortable and vocal.
Generally if a pig is in a pet store cage it will not run around, they live sad and sedentary lives. If you put a pig that is used to a pet store cage into a larger C&C cage, they don't know what to do with all the space and often feel insecure. However, they will adjust and pick up in activity before you know it! They will thank you for it in the long run!
This site advocates a cage size for 1 pig of at least 7.5 square feet, while 10.5 square feet is better.
Also, pigs are social animals and do better in a pair or small herd. have you considered adopting him a friend?
Here are some great cavy care links if you have not found your way to them yet!
www.cavycages.com Great Cage info
www.guinealynx.info Great Care info
www.cavyspirit.com great info on pig socialization and why not to breed or buy.
Welcome to the world of Pigs!