My son started at 11 yrs old. His first clays gun was a Remington 1100 LT-20. We added a 28" barrel that would take chokes. He was a little guy shooting Trap, Skeet, and Sporting. As he grew and his confidence grew he progressed to an Ber AL391 for all three. Currently, he has migrated from the team stuff and shoots only trap - when he made that decision we got him a Citori CXT.
My suggestion - if you have any coaches, school teams, or youth shooting near you contact them. My sons first shotgun experience was with a borrowed LT20. I was lucky it was a good fit and forgiving recoil. Another kid that was with us around the same time did not get a good fit, had a horrible experience and didn't make it past the first practice.
All of my guns were used. This is a great forum and for the most part the trap community are good people. Do your research but don't be afraid of used.
Also - on his 1st 12ga we had the group we bought it from add the kick-ez butt plate. Helped with recoil and made it adjustable as he grew.
If you think it will be something he sticks with - look for one with an adjustable comb and butt plate - the gun can "grow" with him and last longer that way.