RE: ml preferance
I honestly think you did your son a favor by starting him off with a traditional rifle. As for reliving your past, if that's the case, whats wrong with that? Wouldn't you want your son to experience some of the fond memories you have with your early black powder rifles? Believe me the first roundball he sends through a deer and makes a clean harvest is one he will remember and brag about for many years, to anyone that will listen.
Again, just my opinion. He will come to understand that the Kentucky does not look like a modern rifle, and it will probably limit his shooting distances, especially if you hunt with roundball and no optics. What he will come to understand is even something as backward (as some people think) as a Kentucky long rifle, is lethal in the right hands. He will also learn that just because a target is on the outside edge of what might be considered in range, it pays to wait for some better shot. He is going to learn to bepatient.
I was sitting one season with my nephew, hunting over a stand bait. A nice deer came in, at about 70 yards. He was all excited and ready to shoot. But I knew that deer was not spooked, and was going to move closer. So I simply advised him to watch it and be ready. Needless to say he thought his uncle was touched in the head (and we will not argue that point). Well he ended up taking the shot at about 25 yards, knocking it flat. What did he learn... even though you could have made the shot, that does not always mean you have to take the shot. That is something that a traditional muzzle loader taught me. I used to hunt with a 7mm mag and if I saw it, I could shoot it. The trouble is hunting season became more of a morning meat gathering ritual and not a hunt. I switched over to a .54 caliber shooting roundball and it was a whole new world of hunting out there.
The Kentucky rifle will be a real experience for you and him. I have a friend that bought a Kentucky long rifle many years ago in .45 caliber. He's hunted deer for over 30 years with that simple rifle and taken deer almost every year. Last time I talked to him he still has not purchased any inline. As he said, there is no need for one where I hunt.
Your son is going to have a blast hunting with it and you....