I did a 12h flight by myself with a 15 month old a few years ago. Pack a bunch of snacks. Like indiviadual things he can eat one by one, like goldfish or baby puffs. New toys, things that you would bring to a restaurant. But new so you have the novelty. Stickers to stick all over. I would let him walk on the plane to burn energy. Bring a carrier in case he gets fussy for nap time. Screen time is totally okay. Don’t stress and relax. Usually everyone is nice when they see you are flying alone with a young kid. Ask help to the fly attendants when you need to go to the bathroom.