Buy some new stuff along those lines and don't let him have it until you are actually on the flight. New stuff always has more mileage than toys they are already familiar with!
Sticker books, pop up books (that Usborne farmyard hide & seek got us through 3 hours of one flight!), easy puzzle type games. By the time he's munched on some snacks (take snacks!), had some milk, napped a bit you'll be home and dry.
And relax. It's fab that you're preparing so well in advance, the flight will be fine!
I did a 4 hr plane journey with my DS when he was 21 mths. He was quite an easy toddler, in that he liked stories read to him, so we took some Thomas the Tank Engine comics to look at (not the end of the world if they get torn or lost) and are easy to fold up into a pocket quickly if you are using them as a distraction in queues. There are quite a lot of toddler comics around, and often have free toys on the front.
I found that between eating and snoozing, the time could go quite quickly. Try and let him have a little 'walk' down the plane gangway too, you might have to change a nappy at some point which gives you a good opportunity.