It's quite possible that Santa's sleigh helpers are magical and so normal physiology doesn't apply. If Santa can make them fly, it seems a small trick to make the males retain their antlers. As well, not all male reindeer/caribou shed their antlers before Christmas. The various populations shed them at different times and there are indeed some populations where the males don't shed their antlers until January.
It's quite possible that Santa's sleigh helpers are magical and so normal physiology doesn't apply. If Santa can make them fly, it seems a small trick to make the males retain their antlers. As well, not all male reindeer/caribou shed their antlers before Christmas. The various populations shed them at different times and there are indeed some populations where the males don't shed their antlers until January.