Go have a look at the housing market & find another house that you'd like to buy - one that's comparable to the one you're living in. My idea is that you tell him that you'll buy his house at £(insert your offer) but if he sticks to his asking price you'll buy the other house that you just showed him. That should make him realise that he can't dictate the selling price in an open market.

Also, I find its always good to have a backup plan in case he won't drop his price or decides not to sell. And you never know, you might find a house that you prefer!

Good luck.