I.G.
I have been taking my DD to Europe since she was 4 months old, about once a year. If you are flying with him as a lap child get the bassinet seats! My DD fit in them until she was almost two and they were a godsend for me.
Other than that bring plenty of entertainment (iPad/iPod or a laptop with movies, toys, art stuff, etc) and plenty of drink and snacks (I never had a problem with getting kids juice packs, tube yoghurt and stuff like that through security).
I have never medicated my child and frankly would never consider it. Yes, they get restless, just like the adults, so we walk up and down the aisles for a bit...
We are pretty strict about TV time at home, but on the plane I will let DD watch as much ad she wants...
One last thing: if you can take a non-stop red-eye. Here the flight we take leaves at 7pm, that means that DD usually falls asleep after the meal is served and will stay asleep for a good while, occasionally even the entire flight. If that's not possible do not make the mistake of keeping him up for naptime in hopes the he will sleep longer on the plane, from my experience that just throws them off and makes them cranky... I'd rather have a happy active child on a transatlantic flight than a tired cranky one.
Good luck and enjoy!