Airlines probably have that rule to reduce/eliminate their civil exposure for liability. If they won't let you on the plane, you can't sure them when you get into trouble when your passport expires while you're abroad.

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<<< Airlines probably have that rule to reduce/eliminate their civil exposure for liability >>>

They have that rule so they don't get fined by the country when one of their passengers is refused entry

TT26's son's passport will not expire while he's abroad. There's no problem getting back to the US. The only possible problem is that European immigration officials will not let him into the country because his passport does not have 6 months left on it.

If I try to fly to the USA with my Dutch passport that expires in less than 6 months, they will not let me into the country. The airline flying to the USA knows this, and therefore will not let me board with that passport.

Renew it now; you would have to do it anyway when he returns.