He can use a Eurail pass, as he’s not a European citizen. The residence permit doesn’t come into it.

https://www.eurail.com/en/help/interested-in-eurailing/do-i-need-a-eurail-or-an-interrail-pass

He can choose an Interrail pass if he wants, using his residence permit, but that would limit his travel in the UK.

With a US passport they can use Eurail. (No 2 days limitation in the country of residence either, as opposed to Interrail Global Passes).

If they wish to use Interrail (no real advantage) they can but bring the residence permit in case. Staff might want to see your passport to check whether names match but that’s pretty much it.

Slightly off-topic but please be aware that the Benelux Pass does not include travel from/to the UK. If you wish to take the Eurostar from/to London you need a Global Pass (+ mandatory reservations bought in advance as it’s a very popular route with a passholder quota).

Hi all, 

I want to emphasize once again that the residency rule takes precedence over all others. According to our policy, Interrail passes should be purchased by European citizens/residents. If an individual uses a non-European passport to buy a Eurail Pass but actually resides in Europe, they may face fines from the authorities/carriers. It's crucial that we adhere to the rules, and it's the user's responsibility to ensure compliance.

Have a nice weekend!

This doesn’t seem to agree with the advice given on that page. (My emphasis):

If you are not a European citizen, but you are an official resident of Europe, you can also choose to use an Interrail Pass.

That suggests that non-Europeans who reside in Europe - like the OP’s husband - can choose either Interrail or Eurail.

Is that incorrect?

I agree that the words used are not the best, I will forward this do edits. 

Check also the point 1. If you have a European passport and a non-European passport, use the passport/ID of the country you live in. If you live outside of Europe, order a Eurail Pass. If you live in Europe, order an Interrail Pass from Interrail.eu.