Hi Lali -
I've been flying with my now 18-month-old since she was itty bitty. We've done lots of cross-country flights (Cali to Boston), Cali to Hawaii and often with short commuter flights one end or the other, layovers, etc. I haven't flown internationally with her yet though. A well packed diaper bag/carry-on is key, here is what works for us.
-I have a $30 umbrella stroller that we use when flying. It helps us race through the airport & gives my daughter her own seat in the airport. It fits easily in the handicap bathroom stall so I can us the restroom. Its lightweight. It folds & unfolds easily to leave & pick up at the gate, plus if it gets beaten up a bit I don't mind. Since my daughter is on my lap for the entire flight, I think its good for both of us that she has a "seat" of her own in the airport, plus it contains her.
-I also bring along a baby carrier (like a baby bjorn). When we line up to board the plane I put her in it, then fold up the stroller. Now that she's older, I don't always use it but is very handy when I need both hands available for boarding passes etc. Its also less necessary if your traveling with a partner, but if its just baby & you it may be thankful for having this.
-small board books, crayons & coloring book, small rattles, stuffed animals or other toys, plus a few new small toys to surprise him every few hours if he seems bored.
-if you have an iPad/laptop/smartphone, bring along DVDs or download some cartoons to have on hand. Dinosaur Train and Mickey Mouse Club work great with my little one.
Tire him out
-I try to keep my daughter up before boarding the plane. I try to let her walk/run a bit around a quiet part of the terminal for a bit to give her a chance to stretch her legs, get out some energy & tire her out a bit.
Milk & Water
-Have a few empty baby bottles or sippy cups on hand to fill with milk or water, even if you've already trained him to use a real cup.
-Buy a big bottle of water before you board the flight. Flying can be really dehydrating plus lots of people experience constipation as a result. Offer water whenever possible.
-If your strict about giving him whole milk only, you can often get a cup at Starbucks or whatever coffee shop is in the terminal, sometimes they charge you for the milk & other times they'll just give it to you for free. If the coffee shops are closed at that hour, you can usually get a bottle of low-fat milk from the magazine/snack shops. When you board the plane, as the flight attendant to refridgerate it for you. An international flight should be able to do this easily. Sometimes domestic flights don't have a fridge, when thats the case, I ask for a cup of ice, pour the ice into the barf bag, put the bottle/cup of milk in & close it up as tight as I can. Those barf bags do not leak ;-)
-I usually fill a few sandwich bags with Cheerios at home. From the airport snack shops we usually like Chex Mix & peanutbutter crackers and sometimes animal crackers.
-Starbucks & Au Bon Pain often sell bananas or fruit cups with a combo of grapes, melon and berries. These let us offer our little one some fresh & familiar foods.
Extra diapers & extra back-up clothes (for you & him)
-When she was younger, it was very common for my daughter to have a huge diaper blow-out during landing. I think it has something to do with the change in air pressure. I've thrown many onesies away in airport bathrooms.
-More recently we've had diaper leak when she sleeps on our laps, the squirming & unusual positions shift her diaper and suddenly I have a wet lap...
-Don't forget to change those diapers! Airplane bathrooms are rarely pleasant, but make sure to change him when you can to help avoid those leaks, especially if he's drinking all that water.
-Plane delays happen all the time have extras on hand in case your lay-over stretches from 2 hours to 6 hours.
-Hopefully you'll be sitting beside & in front of friendly folks who find your son adorable. My daughter loves peeking over the seat & interacting with the people behind us. If your son is extra fussy, cries for a long period of time & you've got a toddler melting down in close quarters, consider offering to buy a drink for the person sitting next to you (and maybe one for yourself).
Good luck & enjoy your trip.