You don't give him half the value to buy him out, you give him half the equity.
If your house is worth �200,000 and there is an outstanding mortgage of �160,000 then you have �40,000 equity and should give him �20,000.
If you can't afford the mortgage on your own, you will probably need to sell up, he isn't going to carry on giving you his wages for ever.
He may be being nice about things at the moment but you can bet that this won;t last, especially if he has to find somewhere else to live and is useless with money. Go to see the Citizens Advice Bureau.