Be aware of the idiosyncracies of flinters vs cap guns;
1.) For flint you MUST use real BP, which can some times be problematic to find. Not a big deal if you plan ahead for it.
2.) Same thing with flints. You have to plan ahead. Real BP and flints are generally not to be found at the average gun store or big box sporting goods store.
3.) You have to be aware of keeping your flints sharp, and properly oriented and the **** jaws tight. On the range most of us tend to push it until we get a clatch, but for hunting new or sharp flints are a must.
4.) The touch hole has to be kept clear. A little bit of wettish fouling in there will obstruct the flash through the hole.
5.) No matter how fast it is, it is still going to have a longer barrel dwell time than a cap gun, which makes accuracy more challenging, especially from unstable positions. like standing. If he's a newish shooter, and of just moderate skill, this may be a source of frustration for him. Cap guns will be similarly fast to cartridge guns, and similarly accurate that way. So the "frustration factor" may be less.
6.) Flint guns will take about 10% more powder to generate the same velocities due to the gas escaping through the touch hole.
All that being said, flint IS a lot more fun to shoot, and is just as reliable, but it is a bit more finicky to keep everything working consistently. You have to pay much closer attention to protocol with flint.
As to caliber, a 50 will be more versatile, and loads can be kept tame (like 30 gr.). If you NEED to find things in a hurry, the 50 is the most popular caliber, so if they carry BP supplies at all, it will likely be for a 50. The 45 isn't far behind though.