Hopefully by Sunday it will be much better (kids heal fast). If it's not, here's some advice.
If you have a stroller that he still fits into, I would take that. Then, once you get to the parks, go to Guest Services and ask for a Guest Assistance Card that will allow you to bring his stroller into lines with you and have it treated as a wheelchair (that means you would be able to bring it into lines and keep it with you until boarding the ride). If he's much better and you don't think he'll need the stroller in lines, I'd still bring it. WDW is a big place, with lots of walking.
If you don't have a stoller he fits, there are rental places in Orlando that rent out strollers (I think one company is called A Baby's Best Friend, otherwise, look on the Family Board for more help). The strollers in the parks are big enough for up to a small 12 year old, so you wouldn't have a problem renting from the parks, except he would have to walk from the bus or parking area which may be more than he's able.
You might also ask at your resort. I've read that Concierge sometimes supplies strollers for their guests. It's possible the resort might be able to get one for you even if you are not concierge guests.
I would not get a wheelchair for a 5 year old. Because they are made for adults, they are not comfortable for children. The armrests come at about shoulder height, his legs will either be sticking out straight ahead of him (if he sits all the way back in the seat), or if he sits far enough forward to bend his knees, his legs will be dangling and he will have no back support. Also, the adult park wheelchairs are very heavy (probably about 50 pounds empty).
Take some Tylenol or whatever your doctor recommended along with you to the parks. If his ankle is wrapped in an Ace bandage, keep in mind that it might swell during the day (heat and being down). So, check the foot during the day. You want to see nice warm, pink toes, not cool or blue ones.
You can get ice from any of the counter service restaurants. If you need more or he needs to lie down and put his foot up for a while, there are First Aid stations in each of the parks. They are marked on the maps, so you should not have problems finding them. The staff there are very nice and the area is cool, dimly lit and fairly quiet.
have a good trip.