I bet Arnold has tried the hard line with the kid, but learned you only have so much control over another person.
I even remember interviews where Arnold said he was tough with the kids, making them clean up stuff at home, forcing the kids to exercise etc so they wouldn't be spoiled. Seems like this kid just got the Kennedy genetics for a 'soft' body and that's what his body strives towards. One of the other boys was slender IIRC. Although we obviously have control over our weight it's limited to a certain extent. This is true of all genetics... even, or maybe especially, psychological genetics.