I think the way it often works is this (obviously talk to your lawyer):
If he's going to remain on the deed and the idea is you split the proceeds when you sell later, then you pay an amount equal to fair market rent for living in the house. House expenses above that amount should be split equally.
Is this what you want? It protects you against some of the loss if you have to sell at a loss or if the house becomes a money pit. But you also lose half the appreciation if the house value goes up. You have to very carefully consider the condition of the house, your local housing market, the amount of equity in the house, the likelihood that you could qualify for your own mortgage loan etc.
