Does he seem healthy otherwise? Sometimes this can indicate an illness. If he's not his usual self, eating, drinking, playing and using his box normally, I'd take him in for a check up.
If he's acting normally beside the box issue, I'd ask myself if anything has changed in the household. Cats can be upset very easily to any change in their environment.
Usually there's a reason for this behavior. Cats are very clean animals and don't normally want to lay in their box unless there's a problem.
But, if all the above is ruled out, I'd get him a little enclosed cat house that he can sleep in. I'd see if he takes to it first before changing his litter box in any way(ie:taking off the lid).