Are your Disney dates and/or Friday arrival set in stone? If not, I would highly recommend Universal after Disney. If your son is really into HP, it will likely be the highlight of his trip. And FP is really a let down after using the front of the line access at Universal.
That is also an option. We arrive in Florida in the 20th and leave very early on the 30th. My parents live there, so we have total flexibility. We could go straight to my parents' on the 20th. Ride up with them to FW on Sunday. Do Mickey Halloween party or FW activities that afternoon / night. Start DW park touring Fresh on Monday. Head back to their house for a couple days, end with universal the following weekend, and go straight to the airport Monday morning . We had planned on universal first because it's new, and we would be fresh. Yes, my son is a big fan. We asked both kids where they would go on vacation if they could choose anywhere. He said Harry Potter world or china.
