Just get him close to the target and work on form and release before you even think about sighting in the bow. Once he masters consecutive releases will you get a group idea on which direction to move the sights to zero him in.

In ZEN archery, archers learn to draw a bow for many years before they ever even get to shoot, learning technique. The next few years they get to shoot arrows in the dark at a large wall with no spots so they don't even get to aim leaning release, then once mastered they get to actually aim the bow. It's a great way to learn, unpractical but you can do the same thing in shorter intervals.