Tell him you love him as a sinner but hate his sin! That’d be an interesting way to tell him about your beliefs.

Seriously, nowadays being a Christian is very much like walking on eggshells on a floor almost entirely covered with eggs. Nowadays, money, sex and self-will are the three main gods, nobody likes to hear that they are wrong to pursue one of their gods. To make matters worse, many homosexuals seem to feel victimized easily, see intolerance and “homophobia” where there is none, so it makes your position even more difficult, I think. If you’re like me, you’re quiet and like things to run smoothly, you want peace and harmony, rather than confrontation and tension etc. Yeah treat him with dignity, but at some point he’ll probably realize you’re a Christian, and will possibly ask you something like: “But you’re not like those homophobic Christians, are you?” or “Surely, you think God is okay with homosexuality (he is) and sex between two men (he is not), right?”. That’s when he’ll either distance himself or start mild persecution, or if he’s mature and reasonable, he’ll respect your ideas and beliefs.