Boy everyone has jumped the gun on this one!
Boresighting? What if he ain't got a scope?
The basic way is use some cheap slug and start at 25 yards regardless of sights you have. Once you can get a tight pattern move up to 50 or 75 yards. Get set there and move on to the ammo you want to use for hunting. The hunting ammo may pattern a bit differently.
Then see how far you can do justice.
Remember a couple few inches high at 50 will be right close to dead center further out.
Now 2lunger is a dang good shot!!!!