SOmeone will come along with some more suggestions but here is my two cents worth:
Use fastpass to avoid waiting in lines.
The GAC can state that he can wait in a shaded area so that the heat does not get to him. However, there are not always seats in these areas, so I'm not sure how much help that will be. The GAC will not get you to the front of the line. There are other things that can be on teh card. You should talk to the CM at guest services and explain all of his limitations so that they can give him everything that they can to help out.
I can relate to him not wanting the
ECV. It took me 6 months to convince DH that he really needed a manual wheelchair for me to push him in (i couldn't go near the topic of a power chair). Once I won that battle, he realized that he really did need it. It took over a year from then to convince him that a power chair was needed. I could not handle pushing him, lifting it in and out of the car, etc. any longer. Now that he has it, he loves the freedom it gives him. But for a long time he saw it as a loss of independence and a weaness. These are valid feelings and your BF is probably having the same problem accepting the changes he has to make in his lifestyle.
I would try talking to BF again about the ECV. Explain how much more he will enjoy his vacation if he is not exhausted from just waiting in line or moving between rides. The world showcase in Epcot alone is over a mile long. I'm thinking that with your little one you will be in MK a lot. This park is always crowded and lines will not be short. Appeal to his sense of family - everyone will have much fonder memories of this trip if you can maximize your time at the parks (rather than resting at the hotel) and doing fun things together.
Walker Mobility and Care Medical will deliver to your hotel and pick up after you leave. You can rent at the parks, but there is no guarantee that one will be left for him to use. Perhaps he could try it and if he hates it then you could return it early? I promised DH the first time that if he hated the wheelchair he did not have to use it and we would return it. After one day, he decided he could not live without it.
Good luck and let us know how your trip goes. Remember - it's Disney and no matter what happens, you are sure to have a magical time!