You have to put him on a ship just like any other sailor.

When you stopped putting him on ships, he stopped getting XP.

I've had a lvl 9 US Radio Man for like 5 years now...

But yeah, put him on a support or ship slot for him to gain xp. He won't gain anything in the lil red circle.

put him in one of the t slot if you wish to level it to make a squad or fleet but if not, no need to level it up, its only purpose is to make a fleet/squad, ingame messaging, friends list (i can be you friend :D, mine is still blank), requirement in the game in order to join a fleet/squad...

only the coolest nf players have lvl 120 ebve radioman (rumors are that if you get enough vets, the radioman will disable the fog of war!)