I am sure that if the shot was placed correctly a .45 caliber with the right projectile could take down an elk. The question really is if you want to hunt elk why limit yourself to a .45 caliber. Since I am sure you're talking an inline rifle, there is nothing a .45 caliber can do that a .50 caliber can not do better or at least as well. Also in some states the .45 caliber is not legal for elk hunting. They reasoning behind this is with an inline shooting a sabot, you are already down to loading a .40 or .357 caliber projectile to shoot from the rifle. If a person was going to shoot a .45 caliber perhaps a .451 caliber pure lead conical in the 395 grainor bigger range, like those made by No Excuse Conical would be a better choice then a small sabot. The .45 caliber conical has some excellent BC and down range energy.

Personally if elk were on the menu I would go with at least a .50 caliber. The recoil is not going to be any worse then a .45 caliber. Also the selection of projectiles will be much better in the .50 caliber class. The .50 caliber will shoot just as far and hit a little harder which is really what we want with a projectile in the first place. The bigger the bullet the bigger the punch is my opinion. I would personally get your son a .50 caliber rifle and load it down if your concern is recoilas abig factor. Then he would have something to use against anything he wanted to hunt, no matter what state and for the rest of his life.

I do want to go on record that I have never hunted elk with a muzzleloader. When I do, it will be with a .58 caliber traditional sidelock or .54 caliber flintlock, both shooting roundball. I think that would be the real kick, to bag one with those. What ever you decide for your son, good luck.