If you decide to keep it, I can give you a good site to go to!
In the meantime, in addition to what you're already doing for it....HAY!!! Timothy (preferrably) or alfalfa. You can buy it in bags for small pets. They love it more than anything, and it's great for them!
Bless you for trying to help the fella! Guinea pigs are really throw away pets
....people think they're so cute, and then find out that it can be a LOT of work to keep their cages clean, etc.
Do you know there are actually whole SHELTERS dedicated to guinea pigs?! Look guinea pigs up on Petfinder for your area, and that may help you locate a GP rescue near you....
good luck!