Hey!
I am autistic and can definitely help.
A lot of what you're describing is familiar - meltdowns from what appears to be nowhere, refusing to go out, complaining through every outing, and screens being the only thing that settles him.
Here's what's happening:
Autistic children don't have the same filter between their core and the outside world. Every demand, every transition, every sensory input hits directly at the open core with no buffer. His brain takes in more data because it doesn't filter out background noises, lights, emotional tone etc so all sounds are the same level. The screen is the only thing giving his nervous system a break from constant processing.
When you turn it off, he's back to managing an unpredictable world with no protection and no idea how to regulate. Within an hour, he's overloaded. The meltdown is about everything else that's been stacking up with no tools to help himself.
He can't tell you what's wrong because he doesn't know. It's not one thing - it's the accumulation of demands, sensory input, and unpredictability. By the time he melts down, his system is already flooded.
Outings are overwhelming because they're full of unknowns. The Christmas lights walk meant separation from dad, cold (but the coat might have felt worse), lights, crowds, no control over the route or timing. Even if it sounds fun, for him it was just stress building.
One thing that might help for outings:
Get him noise-cancelling headphones or ear defenders. A huge part of why outings are overwhelming is the unpredictable noise - crowds, traffic, background chatter. If he can control the sound input, it might reduce the sensory overload enough that he can actually cope with being out. It won't fix everything, but it removes one massive stressor.
Also lots of time to process before going out, along with clear ideas of what's going to happen when you leave. If the trip is not necessary, allow him to stay home until he has tools to regulate more.
You're parenting a child whose nervous system works completely differently, and you're doing it without answers yet. It's all learning.
I've got a free guide on meltdowns and after-school overwhelm that explains what's happening and what actually helps which can be applied across different areas, if you want:
https://beyond-the-behaviour.myshopify.com/products/pdf-the-after-school-crash
Here's a full booklet on the entire autistic experience too for after if it's helpful!
https://beyond-the-behaviour.myshopify.com/products/autism-beyond-the-behaviour-booklet