First its not all your fault. The driver should be able to line up and get close to the ball with only needing a little left, right help as well as when the ball is under the hitch. Use the outside mirrors to get squared away then a good backer will be able to use the rear view mirror to finish getting lined up. On my truck I look in the rear view mirror and line up the head rest on the back middle seat with part of my trailer graphics on the front of the trailer, at this point I am with in 2 inches of the ball either left or right depending on how I hold my mouth. Wife gives me a little left or right direction and stops me when I am there. The wife and I did have our backing problems and still do sometimes.
I do have a back up camera now and I do say they are marriage savers, best advice I can give is a new camera would make a great Fathers Day gift.
Another suggestion is have him hook up by himself a couple times. Getting in and out of the rig 20+ times to check where you are at sure makes a guy take note of what he should be seeing in the mirrors.
Same with backing, if a driver needs to be told which way to turn the wheel or which way to push the trailer they need more practice. Standard rule when I am backing is make sure I don't hit anything and if I am going to hit something then yell very loud, other then that keep your hands in your pockets and let me do my thing because I know where the trailer needs to go and I know how to get it there it just may not look like it until I'm done.
