We Don't Know How Long We Have
I believe that we all have a finite amount of life in us. One of life's mysteries is that we never know how long we shall live some shall live to a ripe old age, others less so but none of can predict with any accuracy when it shall be our time to go.
I think that you have a little bit more information on when your time will be. But really, if you think about it, you're in the same boat as all of us.
You're Not Selfish
I think it inequitable, that blessed with this uncertainty I am entitled to fall in love, and enjoy my life, while all this ought to be denied to you because of the knowledge you have about the predicament you're in. It unjust and you deserve a fruitfull life like all of us. It is not exceedingly selfish to want and recieve love, because its the most important thing in our lives and you should open your life to it.
Tell Him When You Feel Comfortable
If you love him, then you ought to tell him about your condition and leave it to him to decide. From your letter, this seems like a difficult thing to do because you're afraid of getting hurt.
Why don't you let this relationship run its course for a while, and tell him when you feel comfortable?
But if you love him you must tell him.
I wish you all the best and my prayers are with you.