Look at eating at some of the buffets. Chef Mickey (Contemporary Resort), Crystal Palace (Magic Kingdom), and Garden Grill (Epcot -- actually served family style), all have veggies that I think he would love. And the characters come around to the tables for pictures, autographs. All of these take Advance Dinner Reservations and fill up quick, so you might want to call as soon as possible.
We are taking my 4 yr old in November. We are doing a park in the morning, pool/nap, then a park in the evening for fireworks, etc. Go at his pace and let him do what he wants! You will probably spend a lot of time in Fantasyland, but don't forget about the Pirates of the Caribbean ride (my son loves Pirates!) and Buzz Lightyear rides.
If you have any more specific questions, don't hesitate to ask! I've never taken a 4 year old before, but I've taken over 1000 teenagers! (not all at the same time!)