I don't understand either
They need to sleep during the day as part of their biorhythms. Are you talking about making a bucket wheel for a playpen for out of cage time? I wouldn't cover his wheel - they love their wheel. If he is slowing down as older hamsters do, then just make sure there is plenty of enrichment at floor level in his cage so he can enjoy his twilight days.
Do you think he is declining if he's not very active?