Summer sailings to Alaska tend to have lots of families onboard, so I suspect your kids will be in good company. Just don't plan to see them a whole lot, especially after dinner!
Some folks will be dressed up on formal nights, but probably less so on this itinerary. Your son could probably get away without wearing a tie, but how much extra room will a tie take up? Take one in case he wants to wear it, if all the other young men are getting dressed up. If you take a dress shirt and two ties, he'll be ok for whatever he wants to do. We're sailing the Mercury to Alaska this month and my husband plans to take a jacket and turtlenecks for the dressy evenings.
Other than scenery, you'll be happy about the running commentary broadcast by an Alaska expert when you are near the glaciers, and at other times. Take clothing you can layer for warmth, so you can be outside on deck to watch the glaciers up close and personal. It can be really cold out there, but you don't want to miss it. Take binoculars.
I like the basic tours for a first timer, like the White Pass Railroad up White's Pass in Skagway (DON'T miss the museum of the gold rushers in town), maybe a trip to Mendenhall Glacier from Juneau. We're doing kayaking as part of this second visit to Alaska. Lots of people like the helicopter ride to a dogsled camp, complete with dogsledding, but it's very expensive, e.g. $500 per person. But when else will you be able to ride a dogsled?
Don't worry about duty free shopping. Except for your day in Vancouver, you'll still be in the U.S. when you're in Alaska.