Most allergies are triggered by pet dander. With cats the dander is microscopic dried bits of saliva from them grooming themselves. The Siberian lacks the protien in their saliva that most people have an allergic reaction to. However this is no guarantee your son will not have a reaction when he comes to visit.
I have known people who bathe their cats on a very regular basis (at least once per month, sometimes more) and have had very positive results with allergic people having little to no reaction because they have removed the excess dander. If you get a kitten used to it the bath routine isn't that bad. Maybe the two combined will work for you. Siberians are lovely cats but not inexpensive. You can expect to pay $800 plus for a kitten from a reputable breeder.