I was once an atheist, and no amount of preaching without proof could have brought me to my faith. It has nothing to do with an agnostic or even an atheist having a closed mind. It has everything to do with the "preacher" not having a valid argument.
If he doesn't have proof, he should just drop the preaching and maybe talk about the wonderful ways his life is fulfilled in his faith. After that, there's plenty of time to work with a nice organization like Orthodox Christian Charities :)
I am open to hear his argument but his argument so far has not been very fruitful, so I was merely suggesting something that might do more good. I am not saying he is wrong. He is just not convincing.