HELP! Is my guinea pig sick?

Yes it very much so sounds like a upper respiratory infection. I would try to see if there is a emergency hospital around your way and try my hardest to get him there ASAP! When Piggy's are sick they most likely have been sick for a while and were hiding it, so you have no idea how far it has progressed. Also when they are sick there symptoms get increasingly worse within a short period of time. Deff. take him to vet tomorrow morning if you cant find an emergency hospital tonight. Good luck hope i have helped. Get better little piggy :)