When you buy your sons house do you have to charge him rent to live there?
That's entirely up to you. If you're buying it as a gift, then
probably not. However, it's always reasonable to charge rent to a
tenant, even if you're related to the person in question. You do
not have to charge your son rent. But in the case that you bought
the house you have the right to charge him for any space that he is
occupying.