Some good advice above, some although well meaning ...uhh...not so good. First off you think it's something he will stick with so buy a dedicated trap gun, field guns are out. Model 12 or 870 pump guns will probably be the cheapest and are solid guns, but are not gonna be good for doubles. I have shot doubles with an 870 just for fun but your not gonna be a serious contender. The BT 99 is a solid choice for 16 yards and handicap. Your from missouri, the missouri state trap shoot is memorial day weekend in Linn creek, mo. There's some vendors there and trap guns for sale. Tommy Nichols with guns unlimited out of omaha, ne will have a guy there (Nick) and they will have every browning target gun made there, to pick up and feel.

Talk to other shooters most are more than happy to let you, especially a kid look at and shoulder their guns.Ive seen guys hand 10k-15k gun to a kid and say go shoot it. Just look around a lot before you settle on something. I've got several 870 trap guns, BT99 for singles and a CXT for doubles,, But I like shooting the 870's the best. Most of all I like beating guys with really expensive guns with my lowly old $400 870.😁

Just remember it's all for fun and don't get nervous or let your shorts get tight when you got a good score going! A registered 100 bird much may seem overwhelming to a kid just getting started. So remember you don't have to break 100 targets, you just have to break 1 target, 100 times. Shoot straight, and good luck!

Are you positive he will stick with ATA targets? If so I’d also vote for BT 99 as starting point

But at age 13 he may want to try something else along the way and the BT won’t give that option. One thing you do know is that the fit of the gun is going to need changing over the years. For a versatile target gun but not ATA specific I’d go with one of the other suggestions I saw above being a Beretta 300 or 400 series. Pretty much any gun can be tweaked for fit but with shims it’s super easy with the beretta

not saying don't do it , just saying price your components before jumping in

I remember sitting in the basement with my dad and brother churning out our copy of AA super handicapped loads.

trap master hits on my point there are so many specialty guns and builds on those chassis that the general gun consumer hardly ever even hears about that rule the competitive circuits.

it all depends what level you want to be at

one of my 4-H kids he and his dad got berretta autos not positive the exact model it I know it could take 3 inch shells as he used it for turkey also, he shot 4-H trap , hunted and they shot sporting clays , he could be decently competitive in any club or local with the berretta auto.

now he is out of college and works for a construction company where they have trap shoots and the bosses go on pheasant hunts with customers at game farms.
he went from new hire to #2 trap shooter in the company (I think they let the big boss keep is title #1 so he keeps having them for trap shoots)over the summer and started getting invited along to shoot clean up on the pheasant hunts.

if your boy decides to go all in , what he learns on your 870 will apply but your going to be buying something else later , just figure out what it is first in the mean time buy cases of ammo.

I have priced ammo around me and looked online and can not find it under $8.50 box these days. Our gun club lets us shoot "their" new loads for $7 a box........wasn't too hard for me to decide that one. Every day we take him to shoot costs us $12 a round with the shells for 25 pigeons. He so far has only shot 2 rounds each day at $24. the other 2 boys who are a few years older usually shoot 100 or 4 rounds - $48 a day........
We do reload now and are set up for 410, 20, and 12 but I am not sure it is any cheaper than the $7 loads we can buy at club and we cant shoot reloads for 4H. I do have a spare Mec 9000 in 28 ga with a barrel of old AA compresion formed hulls if anyone is looking. I really dont see us getting a 28 ga in the future.
I will probably just let him choose to keep shooting the 1100 or 870 now and see how he does and what direction he wants to go.

I couldn't beat 7 a box , 7.90 a box was the best I could to this year.
our county does a package so that we don't have to keep track of money every night each kid gets 10 rounds with the program fee.
then if they want to shoot more they can buy a round at a time , but it limits how much we have to collect.

I bet they are on last years supply at 7 dollars that is where we were at for several years.