Sorry, I had forgotten the context - this was the PoA you didn't want. Obviously I've never had to do this. On top of that, ours is an enduring PoA. Yours is both health and finance, lasting PoA.
Yes, he does still need to cancel it, which can be done whether it's been registered or not. Here's a link. I just hope he sees sense:
https://www.gov.uk/power-of-attorney/end