Mabel did exactly the same when she became unwell. We had her for 10 days bounding around and playing then 1 day she slept all day only moving to use the litter tray. We took her to the first vet who couldn't find anything wrong, but gave her an antibiotic injection anyway. The next day she was still the same so we took her to a different vet, who said that she was severely dehydrated and had a raging temperature. So we had to syringe feed her, give her antibiotics and some paste to settle her stomach. 2 days later mabel was back to normal.
I'd say take him to the vet, he could be unwell.
Hope he's ok
Nula
xxx