There really is no "best" firearm for anything and anyone who says otherwise is most likely very young and foolish.
To presume that there exists a universally perfect firearm for self-defense against any or every living thing on Earth is to subscribe to the demonstrably false belief in the concept of a one-size-fits-all firearm that anyone can handle reliably, regardless of the multitude of physical characteristics which differentiates one from another.
In the end, all anybody can do is research the subject thoroughly, then make the choice which suits them best.
That being said, I would also like to state my personal belief that the vast majority of folks on webforums who argue that .44 Magnum is the absolute bare minimum when it comes to viable bear defense cartridges and that folks ought to carry something even more powerful than .44 Magnum most likely have never seen a bear outside of a zoo before, much less ventured into the wilderness where Grizzly Bears roam.
In reality, Bears have been taken with just about every caliber firearm, including .380 pocket pistols and .22LR rifles. Now obviously that doesn't mean that one should go poking around in bear country armed only with a .380 pocket pistol, but it does go to show that bears are not the legendary killing machines which are all but impervious to everything but the most powerful of firearms.
In reality, a good .357 Magnum Revolver with heavy hardcast semiwadcutter bullets is adequate for self-defense against even the largest of bears, hence why 10mm Auto has become such a popular cartridge in Alaska for bear defense, and why some Fish & Game wardens carry as little as .40 S&W. (At best, 10mm Auto is only marginally more powerful than .357 Magnum, and most factory loaded ammo is only marginally more powerful than .40 S&W, which is roughly equal to .45 ACP.)
If .45 ACP weren't enough, then it would have never been able to penetrate that bear's skull, so your son was better armed than many folks would assert.
However, I've said it before and I'll say it again, if I were to venture into bear country, then I would carry my Mossberg 590 Shockwave 12 Gauge loaded with Brenneke Black Magic Magnum Slugs.
Why? Because I can handle it, so there's no valid reason for me to carry less. So carry the biggest gun with the biggest bullet you can handle and leave as little to chance as possible.