It seems to me that as he is contributing materially to the move, plus its his home, its right that he shouldn't have to leave it when you die. I am not a legal expert but maybe you could leave your half of the house to your son for his lifetime or for as long as he is able to live there and set things up so that when he dies, or if he chooses to sell the house, or if he needs to live elsewhere for any reason, then your daughter gets a percentage of the value?