K.C.
I've done 14 hour flights with my three kids (without my husband) many, many times. When my kids were that age, I took them shopping before the trip to choose one new small toy each. Then, that toy went away, hidden somewhere until the airplane was up in the air, and my kids had demonstrated good behavior during boarding, take-off, etc. Of course, making sure the toy made it into your onboard luggage is pretty important! So THAT took up at least 45 minutes to an hour.
Don't forget snacks. And books, crayons, paper to color on. Most of your onboard luggage will probably consist of things to entertain him, actually. And don't forget a book for yourself for that 20 minute nap he'll take.
On those long haul flights, there's a good chance the seat in front of you will have a private little TV. If so, it'll most likely have at least one "kids" channel. I'm not a huge fan of hours on end of TV watching, but on airplanes, we have to do whatever it takes to keep our children quiet and the people around us comfortable. So if that means letting them watch hours and hours of Disney channel on the plane, so be it.
Take him for walks up and down the aisle. *WALKS*, not runs. And go WITH him. Don't let him loose to run around unsupervised.
Yeah, and don't forget the iPad (with headphones). Did I mention the iPad? My kids were very young before we had something as magical as the iPad. Some of those flights WERE challenging. NOW? Yeah, they're older, but still, the iPad (or even iPod) has been a game changer in airplane travel.
You'll be fine. He'll be fine. 3 1/2 is old enough to be somewhat logical. And big airplanes are exciting to kids that age. Have fun on your trip!