I would just leave him as long as he stays happy for the duration that you would ideally like him to be napping so say 1.5hr nap, leave him there til the end of the nap. If he gets upset or distressed then of course go to him, but if he is content just leave him, he will be getting rest time even though he's not asleep as he will not be stimulated by all your normal daily activities.
HTH Helen xxx