It comes down to what you believe is right before God. That's something only you can decide - but you need to work it out clearly so you can explain it to him so he knows why you are 'rejecting' him, if you decide that's the way to go. That's both about gay sex and if sex is only good within marriage.

I'm grateful I've not got such an offer; I'd find it very difficult to resist. As a BL I live a relatively lonely existence, and the reality of a boy choosing to be with me would be very attractive. I hope you can find a way to be a true friend to him as he needs, not according to what he wants.