If you are intent on buying this particular house, try to find out as much information about the property as possible. This includes the house itself, the area around it, as well as finding out as much about the seller as you can. If you can find out the seller's true motivation for selling, his goals for the money received, etc., then that can help you tailor your argument for buying the house at a more reasonable price.
As above members have stated, the local comps are the best objective evidence for your argument. You can also put together a list of everything that needs to be fixed, replaced, etc. with cost estimates. You could then approach him again with a solid presentation showing him the facts as they relate to him and his property. Even if he does not initially agree with the common sense, it will at least get in his head and after more time goes by he might reconsider your offer. Until then, move on to the next potential project and contact him every once in a while. Good luck.