My son learned on a compact 20ga Stoger Condor OU. Sold it for same money when he out grew it.
You mentioned 870…you can always buy a full size and install a compact stock until the full size fits. Then he can still use the gun he learned on.
Most of the guys at our club don't shoot with their first shotgun. Either you bought something that wasn't meant for what you're shooting, it doesn't fit, or it fits fine and you can hit targets but you want to upgrade. If you're planning on selling or trading in what you bought buy something...