Empty parking lot FIRST.
Practice parking, backing in between the lines and going up and down the aisles and turning.
The most important thing to teach him is how to pull away from a gas station pump so he doesn't take out a gas pump when he leaves.
Buy a couple of orange traffic cones and line them up in the parking lot and have him pull up along side of them like when fueling and then teach him out to pull away from the cones without knocking them over.
THIS alone will be a surprising learning curve for him. Better a couple of cones versus a gas pump. :W
Have him keep the cones on board and use them at CG's when you have to back in. Place a cone on each side of the site and how far back you need to park.
Aim for the middle, stop when the end of MH is in line with cones.
You don't need no stinkin spotter when you have your orange cones with you. :B