I am so sorry that this is happening to you. Even though I don't you, I can feel your pain. It's not fair, is it? On your question, speaking of what I would do, I would have to tell the truth. It will come out eventually so it is better to tell him now before you or he falls head over heals. Also there is the responsibility of "safe sex" and caution. I can't advise but I can tell you only what would be right for ME to do. Donno if this helps but what else can I say.
There is no such word as can't. Can't simply means wouln't. Grab as much as life as you can. Future is a long way away for those who don't believe. Don't build a foundation of life on sand. It will take it away with the tide. Love a little more, be unkind a lot less.