I'm on a roll tonight... Kitten question

I got one of the black kittens from the shelter on Friday. I quarantined him in a bathroom, until the girls let him escape the other night. He's not been in contact with Sophie, she sleeps upstairs all night. Since taking him home all he does is sleep and I mean sleep, about 22 out of 24 hours.. He doesn't play, or do cat like things. He just sleeps. His eyes and nose are clear as is his ears. He's eating and going to the bathroom. He does have a soft stool. Any ideas? He is going to the vet in the a.m. It was the soonest appt. I could get. There is nothing going on with the shelter cats at this time- virus wise anyway. I'm stumped. I've googled it to death. Thanks-Kristi