Bill,I'm not saying they don't exist but I'm not aware of anyone that makes a youth o/u.You can always have one cut down though.The best option is probably to pick up a compact semi.Most of them come with spacers and shims so you can change the length of pull.I bought my son a tri star semi when he turned 8.It's gas operated so recoil is pretty soft.It is a little picky with certain shells but as long as he shoots Remington or heavy loads,it functions fine.For the money I paid for it,it's been worth it.He also like to shoot trap so I took him to grices over the weekend to look at some 12 ga guns.Both Benelli and Franchi make compact 12 ga guns.He'll most likely get one or the other for Christmas.
There is really not many "dedicated" O/U trap guns out there especially for youth. I shoot a Browning XT-Ultra Trap O/U and it is probably the most popular double for trap. But it is heavy, too heavy for a youth. If its not the XT and it is a trap gun, more than likely its going to be one of the big names like Kreigoff, Kolar, or Caesar.
I don't know how much shooting he actually does, or if he is serious about it or not, so that definitely plays a role in deciding guns. If its just once in a while thing then a decent field gun would probably suffice. If he is more serious about it a TRAP gun is the only way to go. Field guns are just to hard to be consistent with and are much harder to shoot well with.
My first choice would be a BT-99. If i'm not mistaken there are 3 different sizes of them. You would just have to see which one fits the best. My sister shoots a full size one and she's only 15. They will run you about $1200 though. Cheaper options would be used 870 TB's. Probably around $500-600 Pumps though. Another cheaper yet version would be the new Winchester SXP trap. $350-400 I have heard some mixed reviews on it. You definitely get what you pay for.
Good luck and I can answer anymore questions if you have them.