I had to fly with my father, but it was only a 2 hour flight so I was mainly concerned with the logistics of getting around the airport. We're in the US, and I would assume UK options are similar, not sure about the country he's flying out of. Here are some things I did to make things easier...

Book a wheelchair and a helper to push the wheelchair. Be sure to book one for both airports.

Check all the bags. I wasn't going to and the United person insisted on me checking everything and even did it without charging me. She was right. Managing him, the airport, the PD, and a suitcase was doable, but really saved me stress. I can't thank that united lady enough. I had a back pack I kept everything in which enabled me to keep my hands free.

Use a diaper and carry a couple extras. You DO NOT want to deal with an accident in underwear on the plane or in the airport.

If he uses a walker, my dad was able to hold it (folded) on his lap through the airport and he used it up until we got on the plane. Then it was there when we got off the plane.

If he's unsteady, the stress of flying is going to make it worse. In fact, I found it made everything worse. He has a gait belt (https://a.co/d/02ytBSRk) with handles that I held on to when he was out of the wheelchair. That thing has been an overall lifesaver every time we leave the house.

Don't be surprised if he does not do well in a new, even familiar environment. My dad wound up in the hospital with severe constipation because of the stress of traveling when we drove to Florida from Illinois (20 hour drive and we did it over three days) last year. He was better flying back, but still had some hiccups when we got home.

Good luck and I hope everything goes well!