UPDATE-Possibly sick?

One of my rats, Casanova, just lays around the cage. he doesn't use the wheel and only goes to the top of the cage if he wants food. I've had him and his brother for 4 days and I figure they're about 3-4 months old-ish. He'll stretch out in my hand and rest his head on my thumb and grind his teeth, making a sort of clicking sound. He seems content enough to just chill there but is this some symptom of something our is he just lazy?

Update: Casanova just died sometime between 10:00pm and 10:20pm. Just before 10 I noticed his breathing was really heavy so I put him back in his cage so I could look up the number for the emergency vet, as I was writing the number my nephew started freaking out in his bedroom so I went to calm him down. That done I started to dial the number as I went back to the cage where I found him stretched out on his back on the floor of the cage dead.

I bought him and his brother from Scamps so it was about a 50/50 chance they'd be healthy so I shouldn't be surprised but it still hurts. At least I know he had a good life these last four days.