This might work! The books I want are available in the US Amazon. How do I know if his Kindle account is a US or UK one (I'm guessing it'd be UK)? From your point of view, is it a pain to switch between accounts, or do the books on those accounts all just go into the same pot? thanks!
Well, you can have both accounts that's what I do and I swap between them, if a book is cheaper on the US store I buy it there, cheaper on the UK store, buy it there. It's about two clicks to switch between accounts, it's on amazon in the "manage your kindle" settings, you can change that at will as it isn't your billing address, I use my MiL's address in the US as my "delivery" address there.
They did block me from buying more US books at one point as they noticed I was in the UK, so I got the boy to call up with his best Texan drawl and they unlocked it, but that was a couple of years ago now.
Most of the time the US store is cheaper so I use that, (it also has MUCH better range of books). I haven't used my UK account in months. If he buys his kindle books via .co.uk he has a uk account, if he's paying in $ on .com he has a US account.