Agree with 8x57, the heaviest bullet you can find for it, because we can't all shoot like Franz Albrecht, and perfect shot placement isn't easy on a running boar, and through bushes, & undergrowth ! So whatever bullet, it needs to be robust.
I seem to recall that some countries state a minimum weight, 140g rings a bell, but check.
You don't need to worry about high velocity, or flat shooting rounds, as you'll rarely shoot out as far as 100m, 20-60m being most common.
I've been using a 7x64 /165g for sitting out, and 9.3x62 /255g for driven, both do a very good job.