Is My Guinea Pig Dying?

I was recently given a guinea pig from a stranger that did not tell me the animal had any health issues, we have had him for 3 weeks, When we got him I noticed he was shedding, it seemed to be a lot of shed, well he has lost a lot of hair now, its obvious he is sick, Yesterday he was laying on the floor of his cage and couldn't seem to get up, after helping him up for a few minutes he was up and going again. Today I came home from work and found him in the same situation. He has pee and poop all over him. Ive got him up again but he seems to be having a hard time controlling his back legs to stand up. I have always thought he has had excessive thirst, drinking 1-2 bottles of water daily. However this is our first guinea pig and dont quite know what the norm is. Any idea what might be wrong?