Dongles like that either work or they dont.
Tell him to return for a refund and that is all.
Do not get into any protracted messages with him.
My guess is that the next message you will get is that a 'friend' can fox it for about 50% of what he paid you for it.
Just 'Return for refund' and that is it.
If he raises a SNAD then the same message.
If he returns (I doubt it) then refund.
He has left you positive feedback and had it for over a week so I think it is buyer remorse.