Maybe tell him that you are apprised of the gospel message and that no human being giving you more info will help to "convert" you because you are just not open to considering what they have to say on the matter.
Since you pretty much dismiss fundamentalists regarding their beliefs, why don't you ask God directly to show you if He is real? Maybe try sincerely asking Him. You may feel ridiculous at first because you 99% believe you'd be talking to no one but why not try it? Some people won't do this because they really don't want God to be real. Don't be afraid though...it could have interesting results! 
Also, if you do this, you could tell your friend that you are doing it and that it's up to God (and you) if you are going to believe and that you appreciate his concern but you are going straight to the top so to speak so he need not worry about you or bring it up to you further.
You can also remind you friend that the Lord's great commission is to "Go ye into all the world, and preach the gospel to every creature (Mark 16:15)"...it doesn't say he has to actually "convert" or "win anyone over." Your friend can be confident that he has done as the Lord asked, even if you do not choose to receive Jesus Christ as Lord.
