For those staying off site be there at rope drop. Those who are staying onsite have the advantage of getting into the park that opens early one hour before other guests. Also with
HHN going on a lot if not all rooms onsite are taken.
There are two Harry Potter parks. One at Islands of Adventure and one at Universal Studios.
Diagon Alley is the newest at Universal Studios. I would hit Diagon Alley first. Take a look at the map on Universals site. Ride Escape from Gringotts then get the show at Ollivanders wand shop. They pick someone out of the audience for the show at Ollivanders. If your son is a big Potter fan get him a interactive wand. The map is sold with the wand showing sites in both Potter Parks were he can do magic tricks. Check out the shops, Knockturn Alley and walk through the Leaky Cauldron.
Then get on the Hogwarts Express and head to Hogsmeade in IOA. Do forbidden Journey, Flight of Hippogriff, Dueling Dragons and your done with the rides in Hogsmeade. Lunch at Three Broomsticks check out shops, get a sweet from Honeydukes. Take Hogwarts Express back to Diagon Alley then venture out to other rides in Universal.
Take Diagon Alley in again in the evening it's even more fun. Watch the Gringotts fire breathing Dragon at dark.
Some suggestions for the Potter fans as keep sakes. I made small photo albums with picks from both Potter parks. I bought the Hogwarts Express tickets from the shop at Diagon Alley and put it in the front of the album.
We always get the chocolate frogs and other goodies. Have fun with your planning hope this helps. Welcome back to Universal. It's our favorite destination in Orlando