Fly US to LHR, but return to US from AMS

No red flags for this itinerary, I wouldn't think. I don't think ETIAS will be activated yet; I've read October 2026. But it's a good idea to keep tabs on the process.

The customs officials don’t care, they only deal with the goods he brings into the country. Border police officers who will check his passport won’t care either. After all he’s a legal adult, with a return ticket to the USA.

Btw; the EU Etias won’t be in effect until the last quarter of 2026, so no need to sign for him to apply for that.

I frequently return from a different airport/country than the one I flew into and have never run into an issue. I have once or twice been asked at passport control overseas to confirm what my return flight home was when I've booked 2 one-way tickets - I guess they wanted to be sure I wasn't planning to overstay. But definitely never an issue coming back to the US. They never cared where I'd originally flown into. Sounds like a fun trip for him!!

You should look into buying a “multi-city” (aka open jaws ticket) rather than two one-ways. For example, you would buy DFW to LHR / AMS to DFW. Often this is less expensive than two one-ways.

If you have one-way tickets, it is possible you will be asked to show either a return ticket or a ticket out of the country by either the airline at check-in or the immigration officer on arrival. Some people say this never happens, but it has happened to me. Twice when traveling on one-way tickets, I have been asked about my return ticket by the airline check-in agent. Many years ago when I was in my 20s, I was asked to show my return ticket by the immigration officer in the UK.

Does a trip that flies one way into London and then return one way from Amsterdam to the U.S. cause any red flags for customs?

I think your questions have been adequately answered, but the quote above caught my attention. Is your son really going to be flying on 2 separate one way tickets, and not on a single multi-city ticket? Perhaps as family of crew he is eligible for some kind of special deal?

It doesn't raise red flags, although the one and only time I have ever received the SSSS was coming home from AMS.

I have done multi city tickets on United often. Usually IAD to LHR and AMS back to IAD, The benefit I've experienced is that this multi city was cheaper into LHR and out of AMS because of the lower departure taxes, rather than a round trip from LHR.

Thank you everone! I was pretty sure it wouldn't be any problem, but I just wanted to make sure we had everything ready.

Thanks for the heads up about ETIAS. I had incorrect information because the website I looked at wasn't an official EU website. The official site does state that won't start operations until last quarter 2026.

https://www.eeas.europa.eu/eeas/coming-visa-free-country-and-travelling-europe_en

I truly appreciate all of the help!

No red flags as such, but he needs to make sure he is in full compliance with Schengen rules relating to minimum income, healthcare etc.... as he is more likely to be checked. Young people are more likely to work illegally in Europe, have less money etc..... so they are higher on the list for an inspection.

I'd say the red flag in this post is the Eurail passes. They are generally not a great value vs. point-to-point tickets. And unless they're sleeping rough, they will need to book accommodation ahead.
Here is the Man in Seat 61's take:
https://www.seat61.com/how-to-use-a-eurail-pass.htm

That you are a pilot, can’t your son fly as an S2 or S3 non-rev to LHR & then AMS to the US? I would think as an S2 or S3, he would have greater priority on the non-rev list than those who have buddy passes (S4). Or is your airline different? I would never recommend this of course on or near holidays. I guess a lot of this depends on how old your son is. Is he an independent adult?

I don't see Eurail as a red flag at all, and nor does The Man in Seat 61, if you read to the end of his advice for those under 28.
Even in supposedly pass unfriendly countries like France there are many more non TGV trains and non radial routes than this forum often gives the country credit for. It is not hugely difficult to get around without TGV's, or at least selective use of them.
The same remains true on many routes in Spain.
Germany has overnight fully seated trains on some routes, and I believe Denmark still has as well on the main route north.
At his age there are still lots of overnight trains which have seated accommodation. While you wouldn't want to do multiple nights in a row they are perfectly good for nights interspersed with hotel nights. Most accommodation on all night trains is actually couchetttes not private berths. Far cheaper and perfectly good, if you have youth on your side.
When I was his age and a lot older I went inter-railing (as we call it in Europe) a number of times for 10 to 14 days with barely a night in a hotel normally.
I certainly couldn't do it that way now.
I strongly encourage him to get hold of a printed copy of the European Rail Timetable. That is the inter -railers "bible". You can do and see things with a book that you can never ever do with an electronic journey planner.

The Man in Seat 61 will only get you so far as well.

I'd say the red flag in this post is the Eurail passes. They are generally not a great value vs. point-to-point tickets. And unless they're sleeping rough, they will need to book accommodation ahead.
Here is the Man in Seat 61's take:
https://www.seat61.com/how-to-use-a-eurail-pass.htm

How exactly is that a red flag considering the OP's actual question?
And whether a rail pass is a good value is wholly dependent on an individual's actual itinerary, rather than often repeated rote forum advice. Spending long periods in a couple locations with minimal change of base, minimal rail journeys and purchasing train tickets when immediately available at lowest price, a rail pass may not be a good value. Someone wanting flexibility who may change plans on a whim then a pass could be a great value. I'd bet the house on a random 21 year old turning a $879 3 month continuous rail pass or a $351 7 day pass into an amazing values.

That you are a pilot, can’t your son fly as an S2 or S3 non-rev to LHR & then AMS to the US? I would think as an S2 or S3, he would have greater priority on the non-rev list than those who have buddy passes (S4). Or is your airline different?

I don’t think the OP ever mentioned how his son was traveling. And regardless, yes, every airline is different when it comes to non-rev travel.

I don’t think the OP ever mentioned how his son was traveling. And regardless, yes, every airline is different when it comes to non-rev travel.

Indeed, the OP did not. I was making suggestions as I flew as a non-rev for 16 years. While of course there are differences, there are also similarities among the airlines like preferred days of the week to fly for example. Open jaw itineraries can work out well as a non-rev. I did that a lot.