This is just me. All kids are different.

I raised three boys to become responsible safe (knock on wood) shooters and hunters. If your boy has been used to a BB gun a .410 will be a 'real gun' to him even if not to you. For safety I started mine with single shots. My favorite is the Savage model 220 because it is hammerless with a tang safety making the transition to a double practically seamless. Find one on GunBroker.

I tried to move my boys up to a 20ga too quickly and they let me know they were afraid of it. When you do move up to a 20ga make it a double or a restricted capacity pump; the extra weight will
offset the increased recoil.

Good luck, do it right and you will have the best hunting buddy you will ever find!...Geo