I bet Boris just wanted to settle in a different place. He knew you would find him and build new pen around that better location.

If the distance is too much for you to bear
Once fenced in, you can take your time with a corridor along which you can easily push him but like Brakxel said; but this is only good for short distances.

One thing about how-to not lose all your herd in a raid/to random grounded enemy:
Build secondary but somewhat bigger perimeter around the pen. For example I have few meters between these defensive lines and the mobs don't notice the herd nor is the herd alerted by a wondering mob so nobody aggros at nobody and usually nothing attacks the fence.