Does he feel hot at all compared to his litter mates? is it tiredness or lethargy?
Most vets do a saturday surgery and it's easier and cheaper to sort them out at the start of illness than 48hrs later. Also pets at home have vets, I've found them really helpfull when our cat decides to be ill on a Sunday.
Have they just had jabs? that makes them a bit tired.
My 6 1/2 month old kitten Jelly is doing nothing but sleep , he is eating/drinking & plays in thd garden (wont go any further) twice a day , but he is usually active & playing with the other kittens im going to ring the vet monday , im just really worried Hoping somebody can give me some reassurance as i would be devastated if i lost him