I couldn't imagine picking up a shotgun for the firt time and attempting to hit a moving target. When I got my first shotgun (Rem 870) at age 18, I took it out into the boonies every chance I got and practiced with it. I shot paper, boxes, cans, tree trunks, starlings, water...... It really helped me to understand how the shot was acting through the air, how the shot string looked on target. I took apart shot shells, inspected the payloads, and read any magazine articles I could get my hands on. (internet was barely invented...)
A year later, I experienced my first duck season. I went hunting a lot with my buddy. The limit was 6 birds, and he frequently shot his 6 plus a few of mine, as I struggled to understand the leads and angles of the moving targets. But the time spent experimenting helped alot and soon I was able to hold my own. CONSISTANCY with a shotgun, in my humble opinion, is the trick!
Tell him to stick with it and keep practicing!!! One day at the range it will all come together and that will be a day he'll remember!