Will driving the ECV be hard for him - use up too much of his small reserves of energy? I think a wheelchair might work out better for you all if you have enough adults/older kids to push.
I hope you have a wonderful trip, full of good memories in the making. (((hugs)))
I agree.
Since the OP can see him going downhill in just the past 2 weeks, a wheelchair is a more realistic option. He may want to be in control, but driving an ECV takes a lot of energy, concentration and alertness, which will shorten his ability to be up and around. If he has one and doesn't feel up to using it, you are stuck, since he needs to drive it.
Also, with a wheelchair, it is possible to get ones with elevating footrests.
As was pointed out by another poster, there are attractions where a guest can remain in a wheelchair, but would need to transfer from an ECV to a wheelchair. That's 2 transfers each time - one to go on the attraction and one to get off.
He can go to First Aid in any park to lie down. It may not be the most restful sleep because it's a cot, but if he's worn out, it will help him recharge. You can also go to one one the nearby resorts if you are MK or Epcot and rest in the lobby. Those resorts have large lobbies with comfortable chairs. It would require a trip, but may be less effort than going back to your own resort.
It may help to think of energy as a bank account. The account gets replenished by sleeping and resting. Everything that's done all day has a cost in energy. If you do too much, just like a bank account, you get overdrawn. If that happens, some of the replenished energy goes to making up the overdraw.
A healthy person gets replenished much easier and takes less energy to do things. He might be at the point where just getting up and moving around uses most of the day's energy and he needs to replenish often during the day.
He may also use so much energy that he can't replenish as fast as he is using and would have to just take a few days to rest. Keep in mind that even short days may take a lot of energy just with the coming and going. He may want to go every day, but may need to only go a few days with rest days between.
On our last trip with my mom, she was not considered terminal yet, and was still doing much of her own housework. But, one day at Epcot in a wheelchair was what she was able to manage.
When my friend was terminal and stopped chemo she decided to order an adjustable bed. The medical supply also brought a bunch of other equipment that it didn't seem like she needed yet. Well, one day she could still do the stairs, and then suddenly the next she couldn't. One day she couldn't stand up fast enough to get to the bathroom after it was no problem earlier in the day. My advice would be in addition to any accessible room features to order any medical supplies or devices for the room. He may be fine in the parks one day and then be too worn out to get out bed for basic needs the next, in which case you'll want to be prepared.
You can get medical equipment delivered to your room if you need it. I would suggest CARE because they are a full service medical equipment company.
Also, keep in mind that things can go downhill very quickly.
We get many people on this board who plan a final trip that they don't actually end up being able to take. The POSSIBILITY of a trip might be as important to him at this point as an actual trip. Although it might be disappointing, often the possibility of the trip and the planning is the gift, not the trip itself.
Prayers for a safe journey, whether or not it leads to WDW.