As it doesn't sound like death is in the very imminent future (assuming your son doesn't graduate until May/June), I wouldn't tell him anything until after he graduates.
Let him enjoy his senior year without this burden. He has a lot on his shoulders right now in applying for college and then choosing the college that extends an offer.
Unless something changes drastically, I would wait until after grad night. And, if a change in your condition necessitates telling him, don't burden him with how long you've known as it will only make him feel guilty and there's nothing your dear child could have done to change things.
I certainly hope that your situation isn't as dire as predicted and that something can be done to improve your health.
Just my two cents.