Well, our experience has been since we began flying in the year 2005, my DS always gets patted down after walking through the metal detector. Maybe it is is because he had scoliosis surgery and has a rod and pins in his back, but I'm not sure. They are titanium and not supposed to light up the metal detector. Anyway, he has gotten a pat down in every airport we have been to, but as I said before, they TSA agents have been very kind to him.
And yes, my other son who has Autism but does not have the medical issues as his brother, uses a travel chair also. A funny story to parents like us: one day while at AKL his travel chair's wheel broke off and there was no way to fix it. Maintenance even tried. So, we decided to rent a wheelchair for a series of days in the parks. He was about 16 yo at the time. He would hold my hand until we got through the ticket turnstiles in whichever park we went to and then he would literally run as fast as he could leaving us in the dust behind him. We tried and tried to keep up with him that first day without his travel chair but we lost sight of him quickly. Panic set in. Where is he??? Finally got to Spaceship Earth and looked around and where is DS? Sitting in a wheelchair at the rental place waiting for us! The CM was trying to communicate with him and he just kept saying mom and pointing my way. Every park we went to that trip he would just run to the wheelchair rentals when we got into a park and wait for us. After that first episode, at the different parks we gave him the multi-day wheelchair rental ticket to give to the CM and his chair would be all labeled by the time we got there. DS craves the safety of the moving chair so much and I believe it is because he can actually concentrate on what is going on around him instead of how he moves in space if he is walking. Besides his hearing protector headphones which we never leave home without, we always take his travel chair, too.