Manchester airport is supposed to be very good. I would contact the airport as well as the airline, you can be escorted through the airport in one of their wheelchairs, and it should make security etc. Much easier.
It'll be fine. We have travelled with a similar young man and everyone has been ok. No more difficult than going in to town really.
If he does have a wobble people will have to put up with it. He is part of the general public and they have paid to share their flight with anyone. If they don't like that they can drive or hire a private plane.
I contacted airline carrier last year when flying with DS. We got priority boarding including use of priority boarding check in. I just explained DS needs and asked if there was any reasonable adjustments they could make to meet these.
You'll be fine though. Ds and I are flying to Spain (Balearics) again in August.
We have flown plenty with DS (age 6, non verbal). Airlines have all been very helpful. We get on the plane last and get off first, and come prepared with a big bag of his favourite snacks and an ipad with some new apps on it. I also give him an antihistamine just before we get on, which makes him slightly sleepy.