Why? Reason: The bullet is not totally stable and in a perfect spiral till after 300 yards. Also the round is in the slight climb, level and slight fall between 100-300. The point of impact is more predictable after stabilization in the latter part of its range. The flatter shooting calibers serve a purpose in closer ranges because it requires no hold over adjustment to hit the kill zone for deer hunting. The recommendation by my son is the 308 due to a lot of factors. 1 training and loading info to shoot the 308 is overly abundant. 2 Match bulk rounds of 308 are relatively inexpensive when compared to others in a wider variety. The mass of the 308 will tend to fight off elements such as wind, heat and different environmental pressures than smaller and lighter calibers with more consistent results. The 308 is well matched in barrel length, twist, maximum effective range, bullet weight and loading info because of such a massive amount of testing for so long. I am going to make a guess on the 30-06. It used to be the snipers choice before the 308. Its best range is not 1000 but slightly further due to bullet speed and barrel twist match. Also the extra kick and chamber pressures to generate extra speed make it a little less predictable along with a cartridge that is close to great but can sometimes produce inconsistencies.
I truly mean no offense to your son but this kind of "information" is why we shouldn't expect someone to be a firearms or ballistics expert just because they're in the military.
1)Every bullet from every gun ever fired is DROPPING from the nanosecond that it leaves the barrel. There is no "rise". The only reason a bullet goes "up" is because the barrel is angled up when it's fired. That makes the bullet rise above the AIM POINT but it NEVER rises above the plane of the barrel. Bullets do not "rise". The principle is not caliber or cartridge specific. It is gravity and physics specific.
2)The "mass of the ,308" is irrelevant to it's flight characteristics except as they relate to one single number... the Ballistic Coefficient, BC. A bullet with a higher BC slows down less over time and drifts less in the wind. It make *ZERO* difference if that bullet is a .308 caliber or .264 or .243 or .177 for that matter. The one with the highest BC number will slow less and drift less.
3)The issue of stability is a very minor question. The ONLY difference in stability (all else being equal) between various bullets is in how they are effected by muzzle blast. Flat based bullets spend very, very slightly less time in the blast and the base of the bullet is flat, tending to get a straight push from the blast, whereas boattail bullets have the tail, which "dwells" in the blast for microseconds longer and can tend to slightly tilt the bullet because it's not perpendicular to the blast. The flat base bullet may be more "stable" for the first 300 yards or so because it doesn't need any time for it's spin to stabilize any wobble. Beyond about 300 yards, the boattail's superior ballistic properties take over and they are usually superior beyond that range.
4)I don't know what "training info" is as it relates directly to the .308 but whatever it is will apply just as directly to any rifle.
5)Load data is widely available for any mainstream cartridge and easily found for most any cartridge for which you can get a gun chambered. If you know enough to pick "Cartridge X", you'll know enough to find (or make) the data. Any main stream cartridge has MOUNTAINS of data available.
6)The availability of components is a non-issue. Fact is, sometimes it's better to have a LESS popular choice because those components are still there when all the .308 stuff is gone. Sometimes.
7)I don't know what being "well matched in barrel length, twist, maximum effective range, bullet weight and loading info..." means. Barrel length effects velocity, only very slightly, and nothing else. It's NOT an accuracy issue. Barrel twist rate dictates the range of bullet weights that will work in your gun. There's nothing magical about it. Factory guns in any cartridge will have twist rates designed for the most typical bullet weights in that cartridge. Custom guns, well, you choose your twist rate, so it is what you want. "Maximum effective range" is irrelevant, since the specific discussion is limited to 1,000 yards and any/all the mentioned choices are good to 1,000, many of them better suited than the .308. Bullet weight, as I said, is irrelevant. BC matters, not weight. These cartridges are ALL plenty enough for deer at any range you can hit one. A heavier bullet doesn't make a .308 better if the lighter bullet in a 6.5mm has better ballistics.
I don't want to start this argument again but just watch
this video and think of shooting a deer. Ask yourself what cartridge wouldn't do it at any range.
I know I sound like a jerk but, unless there's something lost in translation between he and you, your son has a lot of opinion and not much fact.