I would see if you can get a child sized wheelchair from home and bring it with you. An adult size wheelchair (which is what you could rent in the parks) would be much too large for a child that young. I've seen young kids being pushed in one, but they didn't look very comfortable - the armrests are too high to use (mid-chest to shoulder height or higher) and the seat is too deep. If the child sits all the way back, the edge of the seat ends up around mid calf, so their knees can't bend (but it would not be enough support for the casted leg). If the child sits forward so their knees are at the edge of the seat, they have no back support and not a lot of security in the chair; I have seen a child 'bounced' out of a wheelchair when sitting like that when they hit a bump.
If you rent one and bring it with, you will also have it for use at the resort. And, you could get one from home that has an elevating footrest so that you can keep his leg up. If you already have a stroller that works, you can get a Guest Assistance Card (GAC) as Michigan mentioned that will allow it to be used like a wheelchair.
There is information about GACs in the disABILITIES FAQs thread near the top of the list of threads on this board.
I agree that it is going to be hot for him and even if you were able to get a GAC, you will still be outside in the heat for long periods of time. I would seriously consider postponing too. There will be attractions where he won't fit well because of the full leg cast. When my yougest DD was 5, she had surgery on both legs and was in full leg casts for about a month (cut down from the hip spica cast she started out with). We were home and it wasn't summer, but it was still difficult to move her around (and she got pretty heavy by the end of the day).
If you do decide to go, plan on doing some homework to see what attractions he would actually be able to go on. If the leg has to be elevated, he would probably not be able to go on many attractions in the wheelchair. You would be safest planning on mostly just going to shows. Also, plan on trips to First Aid (there is one in each park) where he can lie down and rest in the cool.