Even if you weren't facing this health crisis, I still would have encouraged you to do what you could to reestablish contact with your son.
After all, nothing will be gained by continuing to cut yourself off from him. And even if you don't agree with his lifestyle (which, you say elsewhere in your letter, hasn't changed), he is still your son. You may find that he regrets the break between you and will welcome a renewed relationship -- even if it's difficult or awkward at first. The Bible says, "If it is possible, as far as it depends on you, live at peace with everyone" (Romans 12:18). He may reject your attempts to reestablish contact -- but you'll never know until you try.
The most important thing I can urge you to do, however, is to turn to God as you face the uncertainty and loneliness of your illness. We are never alone when we know Christ, because He is with us every moment of the day. Even when we face death -- which comes eventually to us all -- we know we do not face it alone, and ahead of us is Heaven. If you have never done so, turn to Jesus Christ and commit your life -- and your future -- to Him.
Then pray not only for yourself but also for your son, that he will realize his need for God's forgiveness and new life. Christ gives us hope -- both now, and for the future.