Hi Andy. Kids are smarter and more attuned than you think and they worry about things too. My Daughter was 5/6 when I was DXed in 1975/6. Right away she wanted to become a scientist when she grew up, to find a cure for her Mommy's MS. She didn't...she became an Investment Banker instead.
Have that Family talk and let him be the judge when you have told him enough.