I've done 22h flights with both DSs annually, starting when DS1 was 2 and an only, right through to this Easter just gone, when DS1 was 8 and DS2 was 3.
While we've never had an epic tantrum, DS2 has proven somewhat challenging in various ways, not least him passing out through "breath holding" when I was trying to persuade him that he HAD to sit on the seat and put his seat belt on because of turbulence, and him not wanting to. He has a pleasant
habit of NOT starting to breathe immediately he passes out, so that was fun at 36,000 feet. But he came round ok, and I didn't tell anyone, they didn't notice, all was well except for the screaming prior to the passing out.
MOSTLY people are very kind and say how well-behaved the boys are, even when they haven't been - although I have had one arsehole turn around from the seat in front after we'd been sat on the plane about 5 minutes and tell me to "make him stop kicking the seat" - he wasn't!! He was putting his drink bottle into it and had put the tray down, and then back up again. But MOSTLY people are very good and understanding. I find that a pre-emptive "Sorry, hope he's not annoying you" can do wonders to ease potential tensions - people, especially Brits, do tend to then say "no of course not, he's fine, don't worry, you're doing a great job" etc.
For a 2.5h journey, you should be able to take a tablet or even your phone (set to flight mode) and let him watch downloaded shows or films; but do try to teach him how to use headphones prior to that.
Make sure you have something for him to suck on take off and landing to avoid the ear situation - AND a drink, just in case he refuses to eat/suck (DS2 did this last time)
Take a couple of toys/books, whatever he likes, but try to limit the number of pieces that could get lost - so pens/pencils/crayons etc., only give him maybe 2 or 3 at a time. Magnetic games are a great idea! And DS2 really liked these books that work with a water pen
And remember - it's only 2.5h, you'll probably NEVER see anyone on the flight again EVER, and you can do it.