I honestly believe you and hubby will change your minds with regard to the
scooter. Just be prepared with a scooter rental that carries a larger scooter for him. You can call while you are there and get one if you won't do it ahead of time.
Endurance is one aspect, but you also have to consider the speed in which you travel to a certain degree. You will accomplish so much more if hubby uses a scooter. Besides, he will feel so much better using a scooter. Park tickets are expensive and to be able to do more shows, rides, and attractions will pay for the scooter. You might be able to go to a local scooter store and check one out and ride it in their parking lot and all for free.
You can practice at the resort with the scooter; so, he can gain confidence using one.
Without a scooter you may have a hard time with certain shows even like American Adventure in Epcot's World Showcase. There are many stairs to get to that show. Whether or not they will let him use the elevator which is reserved for handicap I can't really say. And how long he will have to wait going up or down on it I can't say. I'm sure they will give preference to those in wheelchairs and scooters for the elevator. Some shows are an uphill ramp climb that can be tough like Fantasmic in Hollywood Studios. You can get a planning video for free Fromm Disney where you can see how some of the rides work.
I think you do have to make other considerations, too. If you are not using a car to the parks can he climb the bus stairs?
I'm not sure about the chairs at the restaurants, either holding his weight. There are some restaurants where it can be tight seating areas and he may have a tough time maneuvering through the place.
You can probably call Disney for special needs and see what they can help you with in this regard. I would do my homework on all the rides you want to attempt with him.
Then, there's the standing aspect for parades and fireworks. There are few seating areas for these events. Any seating areas will be taken at least an hour before the parade or fireworks begins.