As a longtime professional marksmanship instructor including quite a few years of youth instructing I'm a big fan of a good pellet rifle or .22 as a training aid, especially the pellet rifle. The rifle wants to have a good adjustable sight that has accurate click adjustments so that results are repeatable if any sight changes are made. The advantages of the pellet rifle are greatly reduced danger zone, noise and cost. You can fill a cardboard box with newspaper and shoot in the basement if you want.
Linked below are a two articles that may help you decide on an appropriate big game rifle. A properly dimensioned stock and managed (AKA reduced) recoil loads usually make a great starting point. The third article will help if you reload, Yes, I wrote that one but I've been told that it actually makes sense.....
Gary Zinn's article on "Youth" rifles:
http://www.chuckhawks.com/youth_bolt_rifles.html
Chuck Hawk's article on some brands of managed recoil loads (there are more brands out there)
http://www.chuckhawks.com/compared_fusion_lite_managed_recoil.htm
And for the reloaders out there:
http://www.chuckhawks.com/reduced_recoil_H4895.htm
In the "For what it's worth" category, I have used the .223 with success for deer but I don't think I'd recommend it for a beginner. I only use it at closer ranges when noise is a concern and with heavy for caliber Barnes TSX bullets. (The People's Republic of New York does not allow the use of suppressors).
In generally my belief is that making bigger holes is generally better. For example, a reduced recoil 7mm-08 bullet will make a hole a third larger than the area of a hole made by a .223. That's a lot of effectiveness to give up if the recoil is kept manageable and the advantage gets larger as the bullet expands and the wound cavity gets larger.
Lastly, I'm one of the folks who thinks most hunters, especially in the east, have rifle scopes of too much magnification. The 3-9 power scope is by far the most popular rifle scope on the market for 3-4 decades but as the magnification goes up the field of view (FOV) goes down and the apparent wobble as seen through the scope gets larger.
A reduced FOV makes it harder to get on target in some cases but, more importantly, reduces the shooters situational awareness. As we all strive to teach new hunters to always be sure of their target and beyond the reduced FOV is somewhat self defeating in that it makes clearing the area beyond the target significantly more difficult.
Lastly, we all need to strive to reduce our wobble and then accept it when it gets to a manageable level. Higher magnifications have the effect of magnifying the apparent wobble and tend to increase issues such as snatching the trigger as the crosshairs wander past the desired point of aim.
If your farm is like ours I suspect that a scope in the 2-6 or 2-7 power range would work well. My "go to" deer and bear rifle is a .308 that wears a 1.5-6 scope and almost never gets set over 2x.
All of this is of course my opinion so your mileage may vary. I'm glad you are getting the lad out and about and wish you both the best of both luck and memories.
Lance