My twins are just shy of 15 months. They a great walkers, but are really too interested in the world around them to be expected to actually walk somewhere directly. To get from th car to the front door takes about 20 minutes! They're also starting to not want to be put in the stoller, but once we get them in there, things are fine. They're still really happy being in our baby carriers.
Food wise, they eat most "big people" food, it just needs to be cut up into bite-size pieces. I do mix some purees in with plain yougurt but that's the extent of the baby food. If your son will eat grilled cheese, grilled chicken, pancakes and mac and cheese, you'll be covered at WDW. (Of course, at home, my two eat more heathily than that, but I could find those choices pretty much every where). I'll be honest, we took the boys to WDW at 11.5 months, and we worked for the two months prior making sure they would eat what we would find there. It was kind of how I shaped our food introductions!
Whole milk can be found, although not at most restaurants. For example, we went to Chef Mickey's for breakfast and they only had 2%, but in the little counter service next door, they did have little cartons of whole milk, so I ran over and brought those back. I would suggest getting a small cooler pack (we have the Playtex Fridge to Go), so when you do come across the milk, you can carry it with you.
My two are down to one nap a day, and most of our same aged friends are also transitioning to one nap. At this moment, a good touring plan would to be up and out early in the AM for 2 hours or so, come back have lunch around 11, nap until 1:30 or so, and then head out for a little more park time and dinner. Our two have a very early bedtime still (6 pm) and we tried to maintain that. Luckily we were traveling with family, so we were able to go out once the boys were asleep.
Neither boy is talking (one says "that" the other says "car" and "this"), but one of mine also knows a couple signs (milk and eat).
hope that helps!
eta: If you can teach your son to drink from a straw, your life will be infinitely easier when traveling. It took about a week to teach ours, but I was so much happier.