If it is all ready tame, name it so you can tell its your original boar. Build a fence all the way around it with a gate and lure another boar into the fence and tame it too. Then wait until you get 2 one star piglets (the piglets are easier to push) or until you get 2 one star boars that aren't the named one and build a set of parallel walls from the gate to where your farm and animal pen is and push the new one star boars or piglets to your animal pen one boar at a time. Once you have 2 one star boars safely penned at your farm and they start breeding you can get rid of your other boars.

If the one star boar is not all ready tame, build a pen where you want to breed boars with a gate (I like to use short walls and 2 shutters since I can see over them and jump over them and the boars can't) and get the boar's attention so that it tries to attack you and you can lead it to the pen that you built, open the gate and lead the boar inside, close the gate and then jump over the short wall. Once that boar is tame name it so that you know which one it is and lead another boar to the pen the same way and tame it.

Once you have at least 2 unnamed one star boars in the pen you can cull all of the rest and not have to worry about wandering boars.