Hi, not read the full thread but I'm ex cabin crew and ds has severe autism ARFID too.
There are mock cabins in the UK or planes you can visit on the ground so he can see what the inside of plane is like. If you are near Manchester I can give you some good places but you'd have to look for others near an airport you would use.
When you book contact the airline and ask for special assistance. They should offer one person guaranteed to sit next to him free of charge and seats which are suitable (near a toilet, near the door, at a bulkhead) as a min.
Request a suitable room for your needs eg away from the entertainment if he doesn't like noise, on a ground floor maybe if using a buggy or wheelchair etc
Special Assistance can also arrange for him to board first or last depending on what is best for him. They should also allow a mobility buggy, wheelchair etc to be carried in the hold for free and extra luggage allowance if he needs nappies or other items specifically related to disability.
Take food with you in your hold luggage you know he will eat. For us it was breadsticks, haribo and cereal bars. So you've always got something for worst case scenario.
We always take visual supports and I make one specifically for the flight. I google the inside of the plane and ask the airline to provide pics too.
Take a sunflower lanyard. I personally hate them but they are recognised at most UK airports.
Take lots of distraction items ipad etc loaded with apps or programmes that don't require Internet connection. For both flight and inthe airport. Powerbanks too (there are specs which aren't allowed so double check).
Speak to your paediatrician about medication. We've been told piriton helps by our paediatrician at one point but now have other medication prescribed for before the flight.
Make sure your insurance fully covers you (are his immunisations upto date? Most policies will be void if not- check the fine print). They won't likely cover if he refuses but it's best to have.
Albasoil (not sure on spelling), karvol if you can get it and sweets to help with blocked ears.
If he's communicative or verbal can he say what might worry him about flying?
Go for a short flight and transfer if possible. A family friendly hotel so people are less likely to complain.
Best of luck.