R.R.
If you haven't read the books or watched the movies I'm guessing your son and guests won't have either. That's good for you!
We held a Harry Potter party for my nephew when he turned 9 (2.5 years ago) and all the kids had at least seen the movies, my nephew, grandson, granddaughter, sister, BIL, and myself all had additionally read the books, so there was pressure to "get it right."
I watched and rewatched scenes to capture the details as I was the one baking the cake and making the candy for our version of Honeydukes, the candy store in Hogsmeade. I made chocolate frogs, molded frogs I dusted with edible gold dust once set, Hedwig suckers, which were white chocolate owls, Acid Pops, which were Charm Pops dampened slightly with a spray mister, then rolled in Pop Rocks and wrapped in pieces of wax paper twisted closed around the stick, pretzel wands dipped in chocolate with the tips tipped in gold edible glitter, and a couple of other treats. I made up treat bags with some of everything in them for kids and adults alike, and placed them in glass jars and bowls on the kitchen island atop a black tablecloth with a ragged handmade sign my grandson drew.
We decorated with twinkling white lights on the ceiling in the living room, and kept the other lights off to give the party a realistic Great Hall feel. We gave the food names related to characters in the movie like Snape's Snow Cones, Potter's Pizza, Dragon's Legs (BBQ'd drumsticks) etc., and printed labels to go in front of each dish of food. Old books were placed in stacks around the room. Brooms were standing in one corner, anything "wizardly" looking was placed randomly. Etc...
The cake was a castle cake, using rice krispie treats for the outer walls covered with a thinned coat of icing to give them a realistic flair. I used Harry Potter Legos to set up scenes in and around the castle. The loot bags had Harry Potter glasses, jelly beans in flavors from the movie, stickers and other things I can't remember, my sister handled those.
There's still some sites with ideas for your party, I can't give you the ones I bookmarked and used or pictures of the cake and candy, the computer they were on crashed :-/ So I've hunted down some others for you:
http://www.coolest-kid-birthday-parties.com/kid-party.html
http://blog.hwtm.com/2010/04/harry-potter-party-birthday-...
http://remarkable-home.blogspot.com/2012/02/harry-potter-...
http://mjajdesigns.blogspot.com/2011/01/harry-potter-birt...
You can find more by doing a Google search for "Harry Potter party ideas for 5 year old" and feel free to PM me if you have any questions :)