You're going to feel completely differently about your son's diagnosis later this summer than you do right now. Trust me on this. The first few weeks are nuts, but it does get so much less stressful. In a month or so the worry that a low is going to come out of nowhere will be much, much less.
You can search this forum for lots of good threads about things to bring but my top tips will be this:
*Bring twice as many supplies as you think you'll need.
*Test twice as much, especially at night
*Carry all low treatments that you'll need - glucose tabs, juice boxes, pb crackers, etc.
*Use the longer lines to rest, cool down a bit, and treat lows. Your son won't feel like he's being penalized for being low by sitting and waiting.
*Test in line. Any time you feel like it. Give insulin whenever you feel like it. No one will notice.
*don't tell Disney dining about diabetes unless you want to waste half your meal explaining why your son can eat certain things, and why sugar free deserts taste like crap and why they'll make your son have a horrible tummy.
*Do get some good calorie apps for your phone - Fat Secret, Calorie King are the best. Food at disney is still food, and a cup of mac and cheese has the same carbs as a cup of mac and cheese at home.
*If it makes you more comfortable, get a portable travel food scale and portable measuring cups. They make restaurants easier early on.
Most of all, have fun. Don't let the diabetes change how you deal with Disney. Your son will remember this trip and how your family handles it. We just returned from our countless trip since my son was dx in 2005 (also at age 5) and it's honestly become just a non-issue in our lives.
Good luck!