Yep, I agree with all of the above. At 18, you may be surprised to find that he knows more about prostates than you do - many schools teach about male cancers especially testicular and prostate cancer, a lot of unis run info sessions or info campaigns as well. Also, still being young he has an opportunity to make small changes to his lifestyle now that could reduce his risk of problems in the future.
John was 50 at diagnosis, I was 44 and we have 4 children - our son had just gone to university in the Sept and J was diagnosed at Christmas. After a bit of panic and worry, everyone settled back into normal and I suspect that none of them ever give a thought to the fact that their dad has had cancer. To add a bit of context, both grandad had PCa at the time, both grandmothers had died of cancer and we have a genetic brain tumour thing going on as well. He may take it more in his stride than you expect