I have a 4.5 DS and he is a stroller junkie; I can't get him out. Granted he has some tone and muscular issues so we are not pushing him out quite yet. We are leaving on the 27th and we will be bringing both Maclaren strollers for our sons (our younger son is 18mnths). When the 4.5 yr old wants to walk he can and we will just follow him with the empty stroller or put the back pack in the seat but then he has a place to sit comfortably when we are walking longer distances or he's just tired or wants a snack. Its also nice to have a comfy seat when your waiting on bus lines at end of day and for parades or just walking long distances around the parks.
I think you need to go by what your son is used to. Does he ever use the stroller at home? If not then just rent it if he needs one. But if he is used to using one then it might be hard for him to go cold turkey at Disney.