Is my gerbil sick ...?

  His eyes are always closed and he always sleeps. I know he's not dead because he keeps moving around. But I'm a little concerned. He doesn't move around like he used to and the crazy thing is that I've only had him for about 3 weeks. The other gerbil in the cage whoever is always energetic and moving around I don't understand it. I've checked for any symptoms of health problems but he looks fine. He isn't losing his hair, he still eats, and he doesn't have an nasal discharge. The funny thing is when I got him though, was always like that but It seems to have escalated. What could be the problem? Is it just his personality?
How old is he? My 2 1/2 yr old gerbil was just euthanized. He just layed there for 2 days, eyes closed. Would perk up if I nugged him. Tried to eat but wouldnt. Doctor said it is probably kidneys. So I had him put down. My other old one a few months ago had fluid and a tumor in the abdomen. The vet appts were not that expensive. You may want to go see a vet. If your gerbil is young, it could be pnemonia. Maybe you just got one that has health problems. If you tell whoever you got him from, they may be willing to help with the vet bill.