First up; reserve yourself a good block of time, grab a drink and start reading the stickies here on top of the disabilities-section. You will find yourself reading so many valueble info you never even considered until reading about it!
As far as specific questions; tube feeding. Obviously being aware of it's temperature restrictions and Florida weather is important. If on continues feeds; no need to feel restricted to airconditioned areas. There are all kinds of bags out there that keep tube feedings at their safe temperatures. With summer warmth in Florida, it is a smart move to play it really safe and also protect the tube from bag to body itself. There are some aids out there for it, but you can also make ones yourself by using isolation fabric and put together one that fits his specific tubes, possible enlongator he might use etc. Keeping tube out of direct sun will also help hugely.
If on intermittend feeding; feel free to store the feeds at the first aid locations in each park. That way no worry about temperatures. You can feed there if desired, but coming from another tubefeeding user? Great thing about it is that you are nto locked into one location. Once you get the hang of it, you can hook on safely anywhere as long as you can wash your hands (or have wiping clothes or alike on you). Unfortunately some pumps and bags are more sensitive than others, but that's more a case of learning what his stuff can or can't take and go from there.
Now there are special bags for tubefeeding but IMHO they are a HUGE pain, taking up too much space and being very impracticle. Using a normal bag goes much further after some initial sowing. Put in a few straps to hold the feeding bag, if sensitive pump some straps to also hold pump, small hole if need be for the tube to find it's way out and it will work perfectly for feeding. You could even get one size for just feeding and a bigger one for when needing to be about with more stuff.
Transfers; read the sticky, really! Not just info by words but also by pictures. You will also have experience from having done WDW before. You KNOW what certain rides and cars look like. Great beneficial help to translate to the knowledge you have about transfers you can do together or not to size up odds. Also grab yourself an accessible parkmap at entering a park. They will note all types of transfers that might be needed. If not sure about a specific ride; ask at guest services or a CM working the ride. Obviously not all rides will work when needing to transfer with help, but you'll find many rides that can't be ridden without a transfer will be doable when you can transfer with help.
Bags; again, I'm very pro-regular and rather con-"specialized". Not just most of the times much more expensive, but less options and somehow they simply never ever well......... fit my needs, let alone my wishes. (he I AM female!!)
You do mention Permobil, but welcome to this world
there are many types of this brand and they will differ a lot. What might work for a C500s might not work for a C500sKB for instance. Making it more difficult to just broadly compare "whomever also uses a permobil" is how the chair has been outfitted. Seat can differ greatly when it comes to cushions, upholstry, formfitted or not, you name it. What works great for a chair with a thin back might not be an option for those with a chair that has a back that can be as much as 10 inches deep. OK, enough of the negative, here comes the positive; general population of bags. Woohoo! It can take some time to find workable options, but they are out there. Trial and error basically. Depending on his backrest you might find a backpack works great, as long as the straps do not irritate him. Or install a small clip on system to the bar on the back of many Permobil chairs and just click any desired bag on there. Others prefer a bag or at least some storage on their side. Safest then is between your leg and legrest on the side.
Officially permobil does have one (.............) bag that can be used on the side of the chair, but have found it to not fit even standardized chair configuration, let alone anything else. Haven't found anyone yet who does use it (we get it for free as part of how the chair gets delivered, here).
Depending on his likes, dislikes and abilities, others are big users of their footrests. Like myself; my footplate is one bit plate (only type they made with standup function
). Placing my bag in between my legs on the footrest actually helps me prevent me hooking my leg into the other, which I love doing unconciously but is hurtfull. Or if wanting something closer yet not that amazingly heavy; I will hang a bag by it's strap on my empty fold out joystickholder located right beneath my armrest. I've got the option to operate my joystick on both sides by simply transferring it over and thus a holder on both sides. Obviously only use one, so the other is the perfect piece of metal to hang a bag off of.
A lot will vary on what you and both him prefer combined with abilities etc. Many situations can be managed with general run of the mill store available bags, sometimes tweaked a bit yourself with needle and thread to make ones needs or wishes fit better.
Do remember that you can always store medical needed stuff at first aid. For things you dont need on you constantly that can be very welcome if bulky or heavy.
Humming? Honey; that is not your problem. It's not as if he is trying to be annoying or has intentional behavioural issues. He's not even close to possibly harming anybody physically or psychologically. The latter is where I draw a line but before that? It's NOT your, or his for that matter, problem that this comes as part of how his illness presents itself in his case. It's not like he could help it, just like he can't help not being able to run around the park. And yeah, the latter will "upset" some. Just like that is their problem to deal with, so is it when he hums. Unfortunately it's part of his health problems. He can't help it, you can;t help it, it's not hurtfull, it happens. Period. Just like babies will sometimes have a big cry hour right when they are in a line for a ride that will make us hear it all that better. ;-)
I could see how you might think it over, in case it where to happen in a situation where you might not be able to leave, sounds are vibrated back worse, you can be stuck for a longer time and that might make things worse for him. Luckily you know that's triggered by overstimulation. Perhaps you know how to reduce it or where possible prevent it. And if so be the case, if for instance his By Far Most Popular ride would trigger that and would require a long wait in an echo-like waiting line; discuss it at guest services. There are only a few rides that have the option, but you might find being able to wait in a seperate waiting area will help you minimize the effects where possible.
But honestly; let not stop you or him baed on what others might think. People will always think and be annoyed. It's human nature. When something is part of a disability or health issue, you do what you can to minimize and it is not possibly harmfull to anyone; let it never become your or his problem what others think. If you do neither of you will have any kind of life. While growing an appropriate thick skin will be a huge benefit for both of you.