Boars that spawned from a boar stone try to return to the boar stone. Boar stones generate many boars but only the first time you render the stone. If you are going to tame boarstone boars you need to build your pen around the stone or you need to lead the boar back to your pen and make sure that your pen is double or even triple walled. I use a barn surrounded by a low wall surrounded by the base's earthen or stake wall.

The boar stone boar will always be in a corner that is closest to the boar stone and will always try to get back there. When you first load into the area boars are large enough that they can clip through the walls and escape (hence the reason for multiple walls.) Once your boar stone boar(s) produce offspring you cull the original boar. The off spring won't wander far from where they were spawned (which is your barn/pen).