Before we went to Disney, we practiced our "Disney legs" and took very long walks around our neighborhood to build stamina. We did end up renting a stroller while at EPCOT for our 5yo and 7yo. It was our mid-stay park and I thought they could use the break. Since you have some time, maybe you could take your son to the zoo or elsewhere where he would need to walk a lot and you could gauge his limit. Good luck!