I tried to post on the stand alone popcorn thing, but I've been having trouble with the post button lately.
The popcorn is only at Preludes, and it is pre-popped in a bag like you get at the grocery. DS is a big popcorn fan, and he ate the whole bag (about 3/4 at the show, and the rest after dinner). I took one bite, and it was standard grocery store stuff. If your son really wants popcorn, it will be fine. But if he's looking for the stuff like WDW, this ain't it!
As to the price, I do think $2.00 was right. There is no tip added to the snacks, unlike the drinks (smoothies). DD found the rock candy about 1/2 way through the trip, and we gave her the "you can have candy or a smoothie, not both". She went for the candy most times, and it was $1.00. Probably the most reasonably priced thing on board LOL!
When I checked my stateroom bill in the midst of the cruise, it took quite a while to figure out those weird $1 and $1.50 Upbeat charges, but it was the snacks at Preludes.
As far as I know, to take popcorn into the Buena Vista theater, you'd have to go by Preludes at a time it was open and buy and bag to carry with you. Preludes is only open around the shows in the WDW theater.