It used to be that early December was a very slow time at WDW, with crowds building daily until Christmas, however, folks are catching on that this is a good time to go and there is no guarantee for low crowds then anymore. Crowds will be lower if you wait until January, but then all of the great holiday stuff will be over. Personally, I think folks worry too much about the crowds. They are hard to predict so just plan on them being there. Have a good schedule, use FP, etc. I would never promise him low crowds at any time of the year if it were me!