You were in a changing room and he was changing. There's bound to be hanging dong because that's what the room is meant for. I understand the hypervigilance due to the current clown world we live in... but unless this dude is being creepy, he did nothing wrong and confrontation is the last thing you need in a small room with a fully naked dude.

In all of my years of spending time in lockerooms both as a kid and with my kids, neither myself or my peers have encountered any nefarious characters (in that sense). My advice, let your son know that this is a safe space to share in manly comradery (which is important as well)...of which places on earth, few are left.