My guess is that you have not surprised him completely, and he may be charging at you. So I am surmising that you sidestep him and go in from there.
ZULU has it right. I go with a guide, who has 3 or 4 dogs.
The dogs stop the hog, you turn him over by picking up a hind leg. The guide holds up the hind leg, and you reach under the head & grab the ear on the opposite side. Pull hard on the ear & the head will turn away from you.
Stab him in the heart.
One of the dogs is a "specialist" -- he only bites the pig in the testicles. The dog has figured out that this end doesn't bite back.
I know, it sounds like cheating compared with what you were envisioning, but it is still plenty exciting, and people get bitten all the time this way.
BTW, I have seen a lot of hogs shot. Never a 600 pounder, but plenty of 300 lb ones. A .357 will kill with the right head shot. So will a .44, either a special or a magnum. My guide carries a .243 rifle and does not feel "undergunned". A .30-30 is always good, and I have had nothing but one-shot kills with a .308. The aim point is the midpoint between the eye and the ear opening. My friend got a one-shot kill with a .54 cal muzzleloader and a round lead ball. Not exactly a powerhouse round.