Sorry, Nannan2, I didn't mean to read and run but I think the situation with what you need is quite complicated. I think you need to look at the different forms of ownership to determine how to buy the house and still get your other son to inherit.
We bought a house with my grandfather many years ago and we had a solicitor's agreement drawn up to determine all the different things we needed to consider. He was happy with the house as was, we wanted to make improvements so that had to be taken into consideration on inheritance or sale. He wasn't going to live with us so that had to be written in that he had no right to live with us and under what conditions he could force us to sell. As it was, when he got ill, he moved in with us and by the time he died, he had kindly transferred the title lock, stock and barrel so we didn't have huge fights with the executors of his will!