Yes, most importantly take a pushchair, even if you son no longer uses one normally, it will be essential in DLP. He will be a lot less tired, he'll be able to take a quick nap when exhaustion hits, and very importantly, it will be a lot easier for you go to from one place to another!!
It's also a good idea to plan and book some meals - personally I'd recommend buffets since your son will not pay anything (my DD has a really good appetite so it worked out great). It's also good to have the meals booked to avoid going from restaurant to restaurant with a hungry child, looking for a place to eat. We also try to go to eat early to the counter-service restaurants (around 11:30) to avoid the rush of people, overall it's a much more pleasant experience, particularly with children.
Don't plan too much into a day, the rhythm is very different when you're travelling with a child. I find it's better to have shorter days in the park (and go for more days) than trying to cram everything into a couple of days. First day you might be OK but soon you'll have an exhausted, grumpy child with you that doesn't want to do anything (at least that was my painful experience). My daughter is now 5 and we still try to either leave early, or at least give her some time to sleep in her chair. For us going to the Starbucks in the Disney Village works great in getting her to sleep (even if it's just for 30 minutes)
One attraction my daughter has always loved in the labyrinth, I think because it's the one place we let her walk ahead, she loves telling us where to go. If they are open, the playgrounds (in Adventureland and Frontierland) are also great for letting them run around for a bit...