When should a man get screened for colon cancer if his grandpa died of it in his 40s?

Depends: If neither of your parents had colon caancer or polyps, you are probably not at increased risk based on grandfathers history and can wait til age 50. But if either parent had polyps or cancer consider earlier screening with a colonoscopy. This all assumes your are not having any symptoms now.

Ten years earlier: While a grandfather is not a first degree relative it may suggest an inherited gene problem and may be a good reason to seek genetic counseling and testing. Most cancers are still random events, but i would suggest doing a colonoscopy at age thirty for peace of mind. If negative, then age forty.