If you only have one shot, a movie with a variety of heroes have a greater chance for him to like anyone of them

This is exactly why I say he should start with the first Avengers. I hate when people say you have to watch everything before Avengers because no you don't, my first MCU movie was Black Panther, then I watched Infinity and Endgame, and only after all that I went back and did a full binge from Iron Man and still enjoyed the experience so stop being dramatic people lol. If he only has one shot to get his dad into it, he should definitely start with multiple characters instead of just Iron Man, not everybody is gonna like Iron Man the way these guys do.