Think if he can't take out a new mortgage for the full value and buy you out, he'll have to sell up and find somewhere cheaper or borrow the difference from elsewhere. Don't sign anything until it is agreed though.
if you are absolutely desparate and he is dragging his feet and refusing to sell, you can take money out on your mortgage. I WOULD NOT recommend this in most cases, but if you have a lot of equity in the house then you are basically taking a bit of your share in advance (if you see what I mean).
Selling the house is definitely the best option though.