But on the way out he got to Amsterdam by some means and had a flight booked to New York. He sent his mom this postcard from there, probably free from KLM, whose plane it is. Would he have flown a DC3 all the way, or changed to a bigger plane at some point?

Even in 1966 my first air travel went Helsinki to Copenhagen to Prestwick to Iceland to NY. In Iceland we changed to a slightly bigger plane. It might have been a jet.
We had cameras but for some reason did not document the trip, unless I find some slide of the NY arrival.
My dad picked us up in NY and drove us to Minnesota, air travel over land was avoided for the cost!
Any idea what planes were common for Atlantic crossings in 1950?