Hide the scales. You don't need them to measure fitness.
Work on increasing fitness; can he improve his speed running around the closest oval, how many laps of the pool can he do in 10 minutes?

If he's not fat then there is nothing to worry about - he has the build for shot put and weight lifting not gymnastics.

My DH has a strange relationship with food. He often talks about weight and I've noticed my sons doing the same