One of the things that my daughter, who is 4 years old, loved on our trip in April was watching the animals from our porch. We had a 1st floor room with a great view of water and the golf course. We had all kinds of birds come around, plus rabbits as well. Every time we came back to the room and every morning she went out to see what was around. Something simple, but she really loved it. She also really enjoyed playing in the big sand area and playground by the main pool.
When doing the parks, definitely try to enjoy the little things. We have some fantastic pictures of my daughter playing in the water grottos. We even brought her bathing suit one day to Epcot and changed her into it so that she would be able to enjoy it and still have dry clothes to wear afterwards.
Also, many children, including both of mine, get sensitive to the load noises in the rides and/or shows. My daughter was overwhelmed by some of the noise at the shows. We had even bought ear plugs for her to use, but I forgot to pack them. So, we frequently had our hands over her ears for part or all of some of the shows.
Try not to get disappointed if your son doesn't enjoy the things that you expect him to. He'll probably find plenty of things that he really does enjoy.
One other thing that I find people frequently say is "Why do you bother bringing an X year old there. They'll never remember it." My attitude is that even if they don't remember it, that the whole point of a vacation is for the family to enjoy themselves. Also, the kids will remember it for a few years at least. And you'll always have the pictures or videos that you take.
Have a great trip. Bringing young children to WDW can be great. Just try to relax and enjoy!!