You do not need to bring anything to Guest Relations.
I am not sure what you are looking for at Guest Relations. It sounds like he is going to need to use a wheelchair in the parks, which you can either rent there or offsite. For a child, I suggest renting one off-site, since you can get one sized appropriately for him. You can also get one off-site that will let him elevate his leg, which may help with swelling and pain.
You do not need to go to Guest Relations for him to use a wheelchair. There is no pass you need.
As others have posted, Disney's response to mobility issues is to rent a wheelchair, so that would be the route I go.