Does he really want a slim-stocked, long barrelled Kentucky rifle, or just a percussion ML of some kind? Narrow things down, perhaps by looking thru the offerings at Dixie Gunworks (online catalog) with him.
Want high-end and cost not an issue? Get the Pedersoli caplock "Kentucky" rifle. Notably, the Dixie "Pennsylvania rifle" is also made by Pedersoli and both have a fancy brass patchbox. Amazingly, the Pedesoli version is $100 cheaper than the Dixie version! A call to Dixie would explain all that.
The beauty of Dixie's catalog these many years is that it has really complete details on each gun, including twist rate and manufacturer.
Not traditional, but I'd go for the Pedersoli Kentucky rifle because it has a faster twist and is thus more versatile with round ball plus slug. Ditto for buying 50 cal. over .45 if hunting is planned. ( Apologies to traditionalists, we don't know what this boy really wants.)
I vote against a kit, unless you are skilled and will help your grandson make it. Many a Christmas-present kit has sat for years or decades, unmade.
Used and cheap, the old time CVA .50 MLs with set trigger aren't bad, and look somewhat like a Kentucky rifle. A set trigger -- as on the CVA or the rifles above -- properly adjusted, is a tremendous help to good shooting in the field, but can be dangerous unless well-practiced with at the range.
Aloha, Ka'imiloa