With some guys it's a masculinity thing. Since he shows no fear himself, he might expect the same from his son no matter how old he is. Perhaps he feels you are catering to your son's 'weakness' and that bothers him. There are some men who insist on toughness and manliness from their sons and feel this has to be ingrained from an early age. If this is the case it may be difficult to convince him that his viewpoint is rather extreme.

Just adding another possibility...