Butterbeer isn't until book 3 when the trio are allowed to go to Hogsmeade sometimes. Diagon Alley is in every book, as it's where everyone buys their school supplies. You'll see it pretty soon in the first book.
I have not been to the Diagon Alley part of Universal Studios., only Hogsmeade when it first opened. But you'll recognize most of the shops and things in Diagon Alley...Gringott's bank, for example. And Olivanders wand shop. You'll also recognize a lot of other food items. I don't know what all is offered at Diagon Alley, but Hogsmeade had things like cauldron cakes and chocolate frogs and Bertie Botts Every Flavored Beans, all of which are encountered in the first book, I think on the train for the first time. And of course Pumpkin Juice.
My kids struggled with reading the books in English, since it's not their first language, so we watched the movies first to give them a base-line knowledge of canon. We've read the first book in Dutch now and DD read the 2nd one at school I think. How old is your son? (mine are 8 and 10 and they read the first book about a year ago)
I'm a huge reader as well, and my kids read well, but I discovered the first book goes really slow for a child. Not much happens at the beginning and it's confusing. My kids had no patience for it until after they watched the movie. Once they knew what was going on, the books were better because they do give more detail. So I would say start with the movie, and then read the book.
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