My mother had Crohn's, but so far I'm showing no signs of it, and as far as I know none of her side of the family have had it either.

Apparently Crohn's isn't all that common in children and is most likely to occur after puberty. And they've made improvements too in the drugs and so on to treat Crohn's. Even if he does develop Crohn's, it doesn't mean it is certain he will die from it. I don't know anything about colon cancer. If you both learn about the symptoms of both diseases then it might help you quickly recognise them if he ever does develop symptoms.