I agree with what the others have said. I have heard there are a few smaller motels with indoor pools, but for the most part, people forget it gets really cold here in the winter. Yes, it is usually sunny and pleasant to walk the Strip in a short sleeve shirt for short distances, but you most certainly will not want to hang around a pool in a swimsuit during the winter.
Take the boy to Circus Circus and give him an all day pass to the rides there - he won't miss a swimming pool in the slightest.