He'd have to carry something like you stated for the larger animals and something like the Taurus TX22 compact for the varmints.Reading this again, with the concern about weight of the gun and defense against boar or wildcat, I'd just go with a Glock 19 or 23 loaded with a deep penetrating JHP like the Hornady XTP or flat nose hardcast and forgo the small game foraging in favor of packing a few more granola bars and a nice lightweight fishing rod. I can't really think of an option that is both good for boar and wildcat defense, but also good for squirrels and such.
I'd still be concerned with the feral hog with a 9mm. Not saying it can't be done. Just thinking I'd like something more powerful because they are strong animals.
That's why I suggested a 5.56 it can be used for both with the right ammunition.