Rejecting unwanted attention.

An old friend back from primary school recently started chatting with me. It is quite unambiguous that he wants pursue a relationship with me. As far as I know, noone knows he is gay, the problem is that I am a straight man.I have never rejected anyone and I am not too sure how I would approach it without hurting him. He is not being explicitly direct about his intent and that only makes it harder to deal with.I would appreciate any advice.