We started hauling our kid across the Atlantic when he was five. The above suggestions are great. Quick addenda:

Go out and buy nine small toys, one for each hour of the flight. Wrap them up in tissue so it's like Christmas morning and dispense them at one-hour intervals.

Choose toys with no small parts or you will be down on your hands and knees all the time. A compact Etch-a-Sketch is a great inflight toy. A handful of little Legos (worse, a Lego kit that will be rendered incomplete if you lose ONE TINY PIECE ) is not so good.

Game Boys just charged our kid up... not so good for a small space.

Avoid sugar, etc. beforehand and cast a critical eye on the kid's airline meal. On a morning United flight once they gave our kid sugared cereal, sugary donut holes and sugar cookies. Since he's got to sit still for five more hours, that's insane.

Pack only dry foods (Cheerios, etc.). Not yogurt, 'cos someone will be wearing it.

Pack sippy cups to transfer beverages from the galley into.

Pack clean shirt in case kid is sick on self.

Pack clean shirt for you in case kid is sick on you.

Kudos to you for trying to strategize this trip instead of turning a deaf ear to a meltdown. Most kids who melt down in flight are suffering from lack of attention, I find.