Let’s see… Don’t know if they have it for young sizes, but “under armor” brand works well. At 6 though, it may be more… Hmm… The way things are positioned as there probably isn’t an issue of mass.
Um, there is no delicate way to say it. When I dress for riding, I lift the All the bits forward and up and pull the skivvies up snug. I occasionally still hit on a rough transition but nothing painful. That’s just part of being a boy, I’ve nailed then on the motorcycle tank too. Oops.
“Hockey jock” also good(no cup though, cup BAD) but UA better.
Best of luck to him, and I hope he sticks with it:)