Hi, V.! What a lucky child, getting to travel internationally already! (I was in Poland twice in the last few years and loved it, BTW!)
Our kids have been traveling for years, starting at 7 months old! They have been great travelers!
SEATS ON FLIGHT:
You didn't mention if you plan to buy your son his own seat.
Our kids are now 9 and 12, so when we started all this there was no option for the discounted tickets for kids. Back then, we would try to travel when the flights were less crowded. (Ex: at 8 AM for a 3 hour flight within the USA). We would book an aisle and a window where there are 3 seats across and cross our fingers that no one would be in the middle. That worked all but 1 time. We ALWAYS brought our car seat, and we had to check it at the gate just that one time.
Soon after, the option of buying an infant seat at 50% off started, and we always took that option. Yes, we hated spending the extra money, but we looked at it as a major safety issue, and being in a car seat on a plane is the safest way for a baby to fly. (You can research this, as I did.) Also, your baby no doubt is used to spending time in a car seat. We found that our kids were not restless during the flights and were quite content to stay in their car seats, someplace very familiar and comfortable for them. Sometimes they slept on flights, sometimes they didn't, but they always were easy travelers, and I think their familiar car seats was a big reason for this.
If you do not plan to purchase a seat for your son, I still would bring his car seat to the plane; if there's an empty seat, you'll have a free seat for your son, and he'll be comfy in his familiar seat. If there are no empty seats for the car seat, the seat will be taken with your stroller at the plane door, and it will be waiting for you when you exit the plane. When you check in your bags, talk to the agent about seating. He/she may see a better seating arrangement for your family and you can decide if you want to move.
Also, I just returned from a trip to Europe, and by chance flights each way had MANY empty seats. Many of ended up being able to take 3 seats across and sleeping, both going on a 10 pm overnight flight (a Saturday), and returning home on a noon flight (a Monday). An overnight flight to Europe is nice because the lights are turned off for most of the flight.
Additionally, international flights often reserve bulkhead seats for families with infants, and they will put "sleeping cots" in front of the seats, attached to the wall. I don't recall seeing this in the U.S., but I may be wrong. A bulkhead seat will give you more leg room. Often, however, the arms cannot go up on the bulkhead seats, between the seats (at least on the larger, international flights), so as your child gets older you may not want that if you want him to be able to cuddle against you during a flight!
So many airlines to Europe now have individual TV/movie screens, with plenty of kids’ shows. You can check with the airline, online, for the variety. Perhaps they will have Barney or Dora or something else that might keep your son busy from time-to-time on the flight.
STROLLER:
Airlines have a very nice system for strollers! You can take one all the way to the plane door! Sometimes I miss having a stroller with me all the way to the door! What an easy way to carry everything! Specifically, take your stroller down the walkway and all the way to the door, with the baby in it all the way, if you want. When you are about to step on the plane, you'll take the baby out, fold the stroller and leave it right outside the plane door (in the walkway to the plane). (If you have specific questions, just ask before you board.) It is so convenient! I tended to take a small stroller with me for travel, the one that the car seat fit into. I left my large, much nicer one at home, on purpose. Sometimes it would have been nice to have the bigger one, but my advice is to travel light when you can! It’s hard enough to do this with a baby)
NON-STOP vs. LAYOVERS:
I know at least one person wrote you recommending having layovers. I don’t have experience with that, as we always book non-stop flights, even if it means driving an hour or 2 to an airport in another city. If this is what you want to do, make sure the flight is a non-stop flight...not just a direct flight (which may stop but without a plane change), but non-stop.
Although my kids now are 9 and 12 and are great, seasoned travelers, and although money is not flowing, we still opt to pay a little more money for a non-stop flight. It just is so much easier with kids...less chance of ear problems on the plane (with the landing and take off pressure changes), less time to get antsy, less chance of delays that set back the whole day and sleep cycle, less chance to be extra hungry because of delays, less chance to cut into the vacation if there are delays. Of course, a non-stop flight can have delays, too, but the more fights you have on a trip, the more of a chance for delays you have.
FEEDING
Keep your child drinking on take off and, even more importantly, on landing, to avoid any ear discomfort (which would lead to a screaming child). I would hold off on the liquids right before a flight so that my babies would be thirsty on take off. Then I would hold off as much as possible during the flight, so they will want to drink a lot during the longer, sometimes more painful (to the ears) landing process.
I nursed my kids, but not on the flights, although I know a lot of people do do that, which is great. I brought pumped milk with us for them for the flights; as much as I really wanted to nurse on the flights...help with the ear thing, as well as the normal routine...my husband and I felt it more important to keep the babies in their car seats, especially for the take-off and landing...so we fed expressed milk in bottles. (I don't know what the TASA regs are on all that now.)
TOYS AND EXTRA THINGS
I always brought both familiar and new toys and books for the ride there and the ride back. I used to wrap them to add to the excitement, and every so often during the flight to give a new toy. I would keep half of the wrapped toys/books for the return flight.
Also, travel with lots of extras...diapers, milk, snacks, etc...just in case of flight delays. Ditto re an extra change of clothes, in case your son gets sick or spits up…not that I would anticipate that, but better to have some extra clothes than to have a child in wet clothes for hours.
Also, try to relax! Your son will be fine, but if you're stressed and nervous, he may pick up on that and be less relaxed on the flight!
Have a nice, safe flight, and good luck!