I found him sitting in the Morthal Inn. I presume he usually walks around Morthal. Be sure to use a bow and arrow because if you use a sword you will get a bounty, even if you are hidden. It is easiest to kill him inside buildings because less people are moving around.
Hern is very easy to kill because he lives far away from any city. The two living there aren’t tough, although if you are having problems I suggest using a bow and arrow.