For the slugs, stick to pretty much any of the traditional Foster style, just not those for rifled barrels. The sabot rounds are designed for rifled barrels and are a huge waste of alot of $$ in a smoothbore. Whether the slug itself is "rifled" or has "rifling" marks on it really only impart a small amount of spin, if any, on the slug.
Since they're pretty inexpensive, buy alot of different brands of slugs and test them out. I've heard alot of good things on the Remington BuckHammers and Federal Tru-Ball. However, there are a ton of other choices to try (Brennekke, Lightfield, Win, Rem...)
Improved cyl. should be perfect for the normal slugs.
I'll agree with 1187vx2, scope it if you can. Even a 0 power red-dot will improve accuracy, and most hunters will be shooting from scoped rifles later on, so no need to avoid them. I'd look into a straight 4x shotgun scope with plenty of eye relief to avoid a split eye brow......new hunters sometimes underestimate the recoil and catch one in the forehead....oh, wait, we've all done that at least once .
Good luck this fall and keep us posted on your son's success.