I strapped mine into compatible car seats at that age, so much easier. I used to fly 2-3 times a year solo with mine (lived abroad) and it was fine, pre smartphones and tablets too.
My tips are puzzle and colouring books, reading books, basic travel games eg connect 4 or guess who, favourite snacks to distract on take off and landing, if you can get seats where they can see the flaps, that entertained them for landing (dd is now an engineer!) and the little plane on the map on the in flight entertainment system, 10 hours of fun there!
Seriously get him used to wearing headphones, bud ones are provided usually, check if you need an adapter for your own, BA used to need one. Most planes have charging on board now but check , take power if needed. Tablet with favourite things on. And book a window seat. Finally sitting still in non negotiable unless going to the toilet, if you let him out for 5 minutes, he'll constantly want to get out, so keep him in his seat, tell him over the next few weeks this will be the case, let him pack a few things he can use on his tray to play with etc. it really is fine but you just need to be fairly firm, we sit still!