@Gracefulskinny
First of all, I will hold a happy thought for y'all that it isn't as bad as they think right now. I know how disorienting this stage can be; when my dad was diagnosed, things either happened way too fast, or not fast enough at all it seemed.
A few thoughts: Speak directly about the destination, distance & duration of the trip to his medical team, and make sure you find out if there will be any restrictions for the trip. Plan ahead for any medications or personal care items that he may need;
Amazon can drop ship to the Resort, but some medications may be required to be filled locally.
Sun issues can be addressed with clothing (long sleeves, hats, long pants, etc.). Remember that if he is sitting in a chair or on a
scooter for long periods it will be important to think about the tops of his ears, shoulders, hands, arms, upper legs and the tops of his feet for sun protection.
Because his stamina may be affected, plan now for your DH to use a mobility device, like a wheelchair, Rollator or scooter. Far better to have it arranged and NOT need it, than to get there and have to arrange it on the fly. There are lots of places to rent from in Orlando. You can rent scooters and wheelchairs from WDW, but you can't take use them anywhere but in the Parks. Some Resorts are pretty big, so you will probably want to rent from an outside vendor so that he will have the device for use at the Resort as well as at the Parks.
Remember that medical equipment flies for free, so if you can borrow a wheelchair or scooter (or Rollator) for the trip from someone in your hometown, in the US the airlines won't charge for it. It goes in the hold under the plane. (Lots of airports are big and exhausting for the able-bodied - so if you are flying, think ahead about your airport(s) that you will use. Don't arrive with your DH already exhausted just from getting there!)
If you are worried about the kids being freaked out about the wheelchair (or whatever device he may need) just tell them that it's only because you all are so busy at Disney World, and daddy gets tired right now. Most kids will accept that and then go right back to being excited about Disney World. The kids will take their cues from how you all handle it; if you are chill-no big deal about the use of a mobility device they will accept it and go on. If they see you and/or your DH get upset or emotional about it, then they may think that they should be upset too. If your DH is cool with it, let the kids "decorate" the chair or scooter!
Remember if you are going to push him in a chair to plan for yourself - make sure you have good comfy supportive shoes on (you will be pushing up and down a LOT of hills, ramps and inclines, so think more like tennies, not sandals) and resist the temptation to hang a bunch of stuff off the back of the chair; it makes it heavier and more awkward to push - and can be uncomfortable for the person in the chair. Most people can't self-propel at WDW because the distances are so great; the average Guest walks between 3 & 10 miles *per day* at WDW, and only folks who live full-time in a chair can deal with that. People who are not used to self-propelling a chair will find that if they can (somehow) make it through the first full day, the second day they will find that their hands and arms are like limp noodles. Vendors will not generally rent a power chair (or "electric wheelchair) to anyone who doesn't use one in their daily life; they are *very* different to drive, and have a steeper learning curve than an
ECV or scooter.
Don't try to overplan; he may not be up to anything resembling a standard "Disney Day". Consider breaking your days into segments, and returning to the Resort for rest breaks; the kids can enjoy the pool while you and DH sit and relax, and then you can all go back to the Parks again after the break. You may even want to try and plan a whole "down day" between Parks days to let everyone recuperate a bit.
Consider getting MemoryMaker so that you can just approach any PhotoPass photographer and take all the pictures you want! If he is feeling good enough, he can stand for the photos; the mobility device doesn't have to be in the photos if he can manage.
Remember that there is a First Aid station in each of the four Parks, in case you need it.
And last, but not least, if I were in your shoes, I might consider a
Disney cruise instead of the Parks. Mostly because it might be more relaxing, less tiring than the Parks, and you still get all of the wonderful Disney Magic (and characters, and wonderful meals, and great memories) that you would at the Parks.