As soon as you board the ship you will see wait & bar staff selling the soda cards. They give you a sticker to place on your cruise card. You can get a plastic tumbler with the card. I never get it, as it is too small. At the bar I ask for the jumbo sized glass for my diet coke.
Jeans are not considered dress-up even for young adults. I will tell you I saw many wearing jeans on my last cruise (Baltic). I even mentioned it to friends that work on the ship. The enforcement of the jeans policy varies ship to ship. Other times I have seen men in jeans not allowed to the dining room.
IF the jeans thing is a huge deal your son can always go for dinner in Horizon Court. Casual all the time.
Enjoy the cruise planning. It will be here before you know it.
I'm on the Grand next July. Can't wait too