Chuck7,
I talked this over with the shooting instructer with the sheriffs office.

He said he had talked this same question over with several gunsmiths.

And he said it doesn't matter wich choke you use it will NOT hurt the shotgun.
If you take the slug out of the shell and drop it down the barrel with a full choke.
It will just fall all the way through.
Since the bottom of the slug expands to fit the barrel and is soft lead.
It is ok to use any choke you want to.

So tell your son to try different chokes to see wich one has the best pattern.

I hope this helps