What would it be?

Stainless CVA Wolf with a youth stock that can later be upgraded to full sized stock. Add a Blackthorn 209 breech plug and you are set.

Depends on whether you intend to go modern or traditional. Modern inline I'd go with a T/C Impact. Made in USA, inexpensive and stock is adjustable out of the box. Traditional, not much to choose from in a youth specific models but pretty easy to shorten the stock on one. Nice thing about muzzleloaders is you can tailor the load to the persons recoil tolerance with bullet and powder charge.

Find him a Savage 10ML and then modify a youth stock to fit it. If you're going modern why not really go modern?

This is exactly why I'm exploring this option

My advice would be an old SS Knight Disc Rifle. They are inexpensive and easy to maintain. They shoot smooth and accurate. Another thought is an old TC Firehawk or Thunderhawk. They shoot great and are a compact package.

Depends on what state(s) he'd be hunting in. So far as muzzleloader-only tags, guns that are legal for one state may be illegal for another. Depends on what game he'd be hunting because some states have different minimums for different animals.

Study the rules for muzzleloader-only hunts for the state he'll hunt in. Those will tell you a lot. Don't trust the gun shops to know the rules or to only sell what is legal in that state, ours sell a lot of stuff that don't fall within the limits of the muzzleloader requirements.

In my state, I'd probably get them a Lyman Deer Stalker .50 caliber percussion rifle. There are a few things I don't like but it's pretty close to idiot-proof, legal, and generally sufficient. Plus, with a factory recoil pad instead of a metal plate and a short 24" barrel its a good choice for a smaller shooter you don't want to over burden or beat into a flinching machine with recoil.

Tom

Built to fit .45 cal or larger PRB flintlock. Well and properly built it can tip the scales under 7#.

Mitch has it right. A T/C impact inline has a stock option to shorten it's length, right in the new packaged box.

2nd choice would be the CVA Wolf.

If you want traditional, Dixie Gun Works has a line of short-barreled sidelock rifles with names like "cub" and "scout" that are a whole lot easier to tote and aim than long rifles. Various calibers from .32 to .50, weigh about 6#.