Make a palisade taming pit with small entrance, let the boar chase you in there.
Once he'll get in, make a half-wall and jump out.
Throw in some mushroom, raspberry or carrots, and sneak out some distance making sure you're not seen (thus palisade works best).

Wait, taming progress can be checked when you sneak in and aim at the tamed boar, but it will spook it again so keep an eye on the taming pit durability.