I've been a T1 diabetic for 29 years and have gone to Disney more times than can count. From my own experience as a an adolescent and now as an adult, your son will do just fine. Just apply what you've learned thus far.
I echo what's been said about Calorie King. I use it every day (thanks SingleDad for the link - I wasn't familiar with it.
At Disney, and whenever I'm out, I do rely on a little guess work. One thing I learned is that one cup is roughly the size of an adult fist. It's worked for me.
As was said by a PP - TEST!
Stick to your meal and snack schedule as best you can. Bring glucose tabs with you. I always put a couple juice boxes in my backpack (never had a problem when they saw my meter at the bag check).
I tend to run high at the end of a Disney day, but a dose correction takes care of that.
I won't tell you not to worry. I tried that with my mother, even as an adult, and I got 'mom look'! I will pass on to you not to let diabetes rule your trip, remember, you rule it!
Have a wonderful time!