Or better, spend 4 days NOT on WDW property. Where would he like to go? Some place quiet? A place that interests him?
Florida has some amazing outdoor spaces, like beaches, state parks, and national parks. They are relaxing, quiet, and very little planning required. If you went to the beach, everyone could wake up when they wanted, chill, and eat on a flexible schedule.
One distinction I have heard is between travel and a vacation. You want to travel= DO things, stay busy. Other members of your family want to vacation = relax. Your best bet is to give them what they want.
For me, 10 days is also too long. Our best trips are 5-6 days at most. After that, theme parks start to annoy us: rude people, waiting for the bathroom, not getting the foods we like. For example, we can tolerate Starbucks coffee and weak hotel coffee, but neither is what I make at home. After 6 days, it is nice to have a good cup of coffee. We also eat fresh fruit every morning. Nothing special; we eat a wide variety of fruits, but decent fruit a is weirdly had to find at WDW. (Why can't you find a grapefruit or a bowl of blueberries at WDW?)
We've also had many shorter trips to WDW. Three days at WDW is fun. We don't try to do everything, just randomly go on rides as offered to us by G+, and eat quick serve meals. We even sometimes have Mickeybars for lunch!