L.R.
Hi! This has come up on Mamapedia several times in the past month or so, and you might want to search for similar questions and see those answers.
Definitely DO buy him his own seats on all flights. You will be exhausted, sweaty and cranky if he rides on your lap and you will arrive at the destination worn out. Also, it's not safe; if we belt kids into cars by law, why don't we have to belt them into airline seats? Turbulence can rip a small child right out of a parent's arms. Spend the extra on seats for him. Do have him in a car seat in his airline seat too. Be certain to call the airline now, not close to the flight, and fiind out if your car seat fits their seats! Not all car seats truly fit, despite what car seat makers will tell you.
You need to take his car seat if you are going to be in a car anywhere at your destination. It's clunky and inconvenient but you'll need it for safety just as you would in a car at home.You'll need the stroller too but might shop for a lightweight travel stroller that folds easily and isn't bulky.
I have flown internationally with our daughter since she was five months old. Airlines don't ask anyone to put kids on the floor during turbulence, since all children and babies must be secured somehow, somewhere, during turbulence, and they wouldn't allow kids to be out of a seat or off a lap at takeoff or landing, when aisles must be completely clear of everything.
Please do not dope him with Benadryl or anything like that "to get him to sleep." I've seen that advice before and it's asking for trouble. The makers of these allergy and cold drugs have even started saying in their commercials' fine print that the drugs are not intended to make kids sleepy. And it can backfire and make some kids wired and wakeful.
If you're worried about flu be sure he has his flu shot - infants can indeed have those. Be prepared to have lots of disposable changing pads for changing him in airplane and airport bathrooms, and carry Clorox wipes for surfaces in places like bathroom baby changing tables.
Be sure he:
--Drinks (swallowing) during takeoff and landing, to relieve pressure in his ears
--Stays hydrated even though it means more diapers
--Has new toys he's never seen before. Bring small toys to distract him. New ones hold their novelty a few moments longer than old ones, but have his best "security" toy along as well.
Have fun. It really is doable.