I'm so sorry that your Husband had to endure that!
We travelled to Disney World right after my Husband's surgery, Chemotherapy & Radiation while he was still very weak. I just asked him what he thought helped him during that time.
He said that the most important thing was that the whole family really went slow and never made him feel rushed. Sometimes he could only go from bench to bench, but he insisted on being there and enjoying the sites of the Parks. I had to learn not to baby him too much and risk hurting his pride!
You could also try to get a Guest Assistance Card at Guest Relations in the Parks. This card will let the Cast Members know that you may need assistance in the Park. He shouldn't need a Doctor's note to get this, but I would bring one just in case. We did not use this, but I wish that I would have known about it then!
And what's great to know is that you have the option of renting a wheelchair or electronic vehicle at the Parks! If you get there and he really is pushing himself too much a wheelchair could save the day!
Going slow through the Parks was really a joy for us and changed the way we see Disney by noticing details that we usually rushed by. I hope that you have a wonderful and healing trip to Disney!