When picking a shotgun for a youngster, remember the growing years. This means a reliable model that the stock can be swapped out as he grows. My son grew up on a standard Remington 1100 swapped to a youth stock, then changed buttpads until he got back to the “grownup” stock. Same with my daughter with a Rem 870. A good O/U with same stock change out routine. I like S/S shotguns, but might be challenging for a beginner.
Also look into the difference between a drop comb and parallel comb stock for trap. My son shoots bunker trap, and almost all guns in this sport has stock parallel to the barrel rib. I think in ATA trap drop combs are standard.
The key is don’t buy so cheap in quality that the gun is aggravating to use, or too expensive that if your son loses interest you regret the purchase. As far as the second part, scan bulletin boards at ranges to find those shotguns other parents bought, now want to unload.