One consideration is the final cost. Kits can often be more expensive in the end than if you just bought a factory finished rifle. Stock finishing supplies, special tools, and the cost of replacement parts if major mistakes are made add up, and kits cost only ~$20 less than a finished gun. Also the quality of the finished gun may not be as good as expected in the end, as some kits just aren't perfect and there can be fitting problems.
Another suggestion would be to determine what the length of pull should the rifle be to fit the boy. Will an adult stock fit him or does he need a youth model?
Some of the optimal models might not be available in kit form.
Depending on how much that you want to spend, you might consider asking Deer Creek Products what they have available in a short stocked model, kit or factory finished. They bought all of the remaining CVA sidelock inventory, and carry Traditions models plus much more.
They are in Waldron, Indiana and only have a phone number & catalog: It's 765-525-6181.
I don't have their 2007 catalog, but in 2006 they listed a CVA .36 squirrel rifle, a Traditions .45 Deer Hunter kit or finished rifle, a youth inline and youth .32 sidelock w/12.5 inch LOP, Traditions .45 & .50 Kentucky kits, .50 Deerhunter kit or .50 St. Louis Hawken kit, .32 & .50 Deer Hunter combo. kit, plus other carbine length finished sidelock rifles and accessories.
Lastly, a .50 caliber rifle will weigh less than the same rifle in .45 caliber. So if buying a gun that might cause an excess weight issue, .50 caliber would be an advantage for a youngster. The powder charge can be easily downloaded so that there won't be any excessive recoil at all for a 12 year old.
Good luck and let us know if there's any other ways that we can help you spend some more of your money!