We just flew with our DS4 in April for the first time. He has high functioning Autism, with some high levels of anxiety about a lot of things. We had a trip to WDW planned, but had to cancel it because last year, he was EXTREMELY scared of airplanes and was very vocal about NOT wanting to fly on one (and we live in California, so it would be a LONG flight). So, we scrapped the WDW trip plan and decided to hold off on that.
However, he came around to the idea of a plane ride, so we figured we should keep it short and semi-local, just in case it went horribly. We flew from Orange County to San Francisco, via Southwest. We appreciated the ability to sit wherever we liked, and from various posts on here, I got advice to sit in the BACK of the plane. This was excellent advice for many reasons! No one walking past us every few minutes, easy access to the bathroom and flight attendants if we needed anything, no one behind our seats, in case our kids got wild (which they didn't, but I was nervous they might). Plus, it was very quiet back there, since no one wants to sit in the back, so most people sat up front, and the plane was not full, so we had few neighbors, and we could stretch out a bit.
Long story short, our son did BEAUTIFULLY. Much better than I had anticipated. He did choose to bring his HUGE stuffed dog from home, which is his nighttime comfort object. He had to carry him on the plane, since he didn't fit in the suitcase.

He hung onto that dog like it was a life preserver, but he was very quiet and well behaved during the flight. I think he was taking it all in. He did very well with the security and airport time. We didn't do much to prepare him for it...we just sort of told him about it day of, and gave him a run down of what we would be doing "next" as it came up. He has issues if you prepare him for something and then it doesn't happen EXACTLY that way, so we have learned to NOT get too in detail with plans, in case stuff does not happen that way.
For example, we drove to the long term parking lot, and then told him
"next we will ride a little bus to the airport" (the shuttle)
At the ticket counter:
"now we have to wait in line to get our tickets (boarding passes) and put our suitcases on the scale"
At security:
"now we have to scan all our bags and strollers, just like at the supermarket"
After security:
"now we're going to buy some water and snacks, use the bathroom, and then wait until it's time to get on the plane."
He did remarkably well with this sort of 'play by play'. We continued this during the plane ride itself, during taxi, takeoff, landing, de-boarding, etc.
For the return trip, we didn't have to really do this stuff, as he remembered from the previous trip and HE was doing the 'play by play'. He has a remarkable memory, and once he does something, he's got it down.

He actually fell asleep about five minutes into the return flight, and slept most of the way.
The only part that was a bit rough for both kids was the descent and ear pain. We had dum dum lollipops for them, as well as gum for the older one, but it didn't help. They both panicked and started crying. My DS with Autism was very close to freaking out mode...he kept clawing at his ears and saying "what's that in there?". I felt so bad. I don't know of any way around this, though (he would never tolerate those Ear Planes in his ears). I kept trying to get him to swallow, but he was so distressed, he wouldn't drink, chew gum, or eat the lollipop. All I could do was comfort him and assure him that it would go away soon.
I'm sure your son will do fine. We did not tell any staff about our son's condition, because we really felt that he would be okay. However, we were on a SHORT flight (less than an hour). If I were taking a longer one, I probably would notify them upon checkin, just in case.