A.C.
Try to secure bulkhead seats, it will be easier to maneuver without the seat in front of you reclined into your space. It sounds like your son may be too old to make use of bassinets that are often available on international flights. Call the airline and let them know you are travelling with an infant on your lap...they may help shift things around.
Pack "diaper change kits" in ziploc bags that have a diaper, wipes, and clean onesie to put in your carry on. You can also pick up sample size diaper cream to include if you need it. It makes it easy to grab what you need and head to the bathroom for changes. The ziploc bags are also good to contain anything yucky.
Your son will likely sleep most of the way if you have a night flight. I found that my Moby Wrap came in handy on flight with my daughter. It made sure she was secure in case I fell asleep. An umbrella stroller may not cut it for your trip...so it is good to have something compact that makes it easy to tote your little guy around.
Buy the washable, triangle shaped Crayolas...they don't roll away. I also sometimes pack washable markers (your son might be too young for this) and let my daughter color on me and herself. This is a last-ditch effort if a meltdown needs to be averted and they come off easily with baby wipes. She is old enough to understand this is only for the plane.
Pack a basic medical kit: Tylenol/Motrin, thermometer, neosporin, band-aids, teething tablets, etc. There is not 24/7 access to everything like there is here in the U.S. Try getting medical care in a small town in Sweden on a weekend...not happening...lol. It depends somewhat where you are going, but assume you won't be able to get some things. Also diapers are MUCH more expensive in EU than here.
I agree that it will b easier for him to have his own seat. I have always bought my daughter one.
Have fun! What a great trip!