Just find him standing there, just kill everything that comes up the hill. There will be some waves and then a bigger mob and after that they are all dead he'll think you are cool and get a ! and he wants to join you.
If the information in the accurate and helpful posts above doesn't work, and you find you're standing near him for a while with nothing happening, then try and server hop since he could just be bugged on your server. I had to do that once to get him but that was a few weeks ago so he's probably good to go now.