when our first boxer, Norton, was young we made a game out of teaching him to come to us. We would sit on the floor at opposite ends of the room and then one of us would have him sit with us, and the other would call him in a happy fun voice "Norton, here!" and as I called, my DH would give him a little nudge toward me and when he ran to me I could give him a treat and go crazy over him and my DH would immediately call him back and do the same--we would have Norton running back and forth between us, totally excited and happy because it was a fun game to him--running back and forth when we called to get a treat and a bunch of silly kisses and petting!
You could also put him on a long lead and as you give the recall command, gently begin to "reel" him in at the same time--praising him the instand he responds by coming towards you. Once he reaches you--lots more praise and a treat!