I am questioning spending $550 for us to go for one day.
Well then...spend a little bit more and take 2 or 3 days there. Make it really worth your while!
DS was 6, not 5, on our first trip in December, and he had a BLAST. None of us went on anything that he couldn't have gone on (I went on Forbidden Journey but he didn't want to because he's only seen up to Azkaban, and the Dementors scare him), but he still had a great time, as did we all.
Hippogriff was just about his fave ride; on our last day he made me go on it with him 14 times in a row.
He also loved the Jurassic Park section (and I'm not sure that viewing the movie is even necessary for a young boy...all they need to know is *dinosaurs* and they tend to be happy LOL), Seuss, just everything. Most of the things he didn't go on were because of emotions...the idea of the Jaws ride scared him too much, that sort of thing.
So I know my guy is a year ahead of yours, but maybe that can help.
I think that it's just such a fun place, with great rides and beautiful scenery (coming into the entrance of Islands of Adventure took my breath away!), great theming (especially in the Wizarding World, especially for my son), that your whole family will enjoy it, even the 5 year old.
Something neat about Universal that is different than WDW...the "rider switch" that WDW does is mirrored at Universal...but where WDW almost always splits the party *outside* of the ride, at Universal the family can wait in the line, and the non-riders go off to a room quite close to the seating area.
So definitely consider going for perhaps 2 days...like I said...make it really worth your while. (imagine how you'd feel if someone said they were only going to WDW for one or two days...painful, eh? because you know how much there is to do there, and how their trip just won't be as good if they skimp on it? that's kinda how a one-day visit to Universal can feel to those who have been there... worth doing, but definitely worth doing really really well if oyu can't do it longer)