No matter where you stay, plan to spend a surprising amount of time at your resort pool.
Ask to ride in the front of the monorail. (Not sure if there is a minimum age for this.)
Boats! There are boats all over the world, and he will enjoy every one of them.
There is a petting zoo at Fort Wilderness.
Always carry a change of clothes so he can play in the fountains which are designed for children.
The atrium of the Contemporary Resort, with the monorails running through it, is even bigger and more impressive when you are only three.
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Pay attention to how he is moving and talking, and not just his mood. If you can realize that he is getting tired BEFORE he gets into a bad mood, then you can take steps to avoid the meltdown.
Remember that with little ones, getting a lot done is the LEAST important part of having a good time.
We took our youngest when he was just under 3, (and still free), and we all had a great time!