At the airport, could you use the family toilet, if they have one? These are usually single stall and large enough for a wheelchair and helper.
Failing that, I'd take him into the ladies' and use a disabled stall.
On the airplane I am sure there is a way to open the toilet door from the outside. If not obvious, ask a flight attendant.
I'd book extra help at the airport to make things quicker and easier for you. I think this is done through the airlines and must be done in advance, but I'm not certain. I would say yes to the wheelchair if he will accept it.
Take at least one complete change of clothing, maybe two extra tops, and wipes, and heavy duty plastic bags. Even if he's not incontinent and doesn't have that sort of accident, there's always the possibility of spilling food or drink and you'd want dry clothes.
Remember not to check anything that can't get lost, especially medicine.
Pack snacks!
I'd have a note ready to hand to everyone, briefly explaining your husband has dementia, can't answer questions, and to talk to you, or whatever the case is, if he would get confused/upset by you saying this in front of him.
i would guess that going through airport security would be most difficult, depending on how well he can follow directions.
If applicable, I hope you have travel insurance.
Also advice here:
https://www.alzheimers.org.uk/info/20030/staying_independent/26/holidays_and_travelling
Safe travels!
