>> book a return ticket for him when prices looked good and then pay the change fee to push the return flight further into the future
This rarely works because if it's a super cheap ticket, you may not be able to find available space at the same fare class ("low price") when you want to change the return leg. That means you are stuck with the original date, or you have to pay change fee plus fare difference(could be hundreds $) to upgrade to a higher fare to get the new date you want.
So one-way isn't necessarily a more expensive option. Often you can find cheap one-way fares connecting through some exotic countries in the middle east, for example.