My very young mouse is dying?

I have two pet mice and I'm pretty sure one of them is one death's doorstep...he's been scratching himself raw for about a day or two and when I fed he and his brother today, he stayed very still, buried waaaay underneath the blankets I have in their cage. He was really limp and a tad chilly. The first thing I did was put him in a box by himself with food and water and a rag for him to snuggle in, and then I cleaned out their cage. He's eating his food now and seems a bit more active, but doesn't seem like he's going to make it. What should I do?! I just got the mice about a month and a half ago from PetCo and I'm not sure how old they were...I ume they were young, though. /: