Just suggest it like, "Dad, a lot of people get pain relief from cannabis, you know, when nothing else words. Give it some thought." You could also show him some of the medicinal cannabis info from the NORML site or Erowid. That way you plant the idea in his head, provide him with balanced information instead of just your own personal pro-cannabis stance, and you still maintain your own privacy. At least you can tell yourself that! My guess is that if your dad's as sharp as you say he is, he probably knows more than you think. . . .