Originally posted by Suzaku:

Create a fenced/walled-in area to contain the boar. Throw raspberries at it once it gets caught inside. Walk far enough away that it can't see you. It will then calm down, eat the food, and begin taming.

It takes roughly 30 minutes to tame a creature, and you need to be in the general area for it to count (close enough that it doesn't visually despawn).