Originally Posted by Xassy:
“Are you cleaning the carpet properly? It's possible they can smell it and it's become their patch. Dogs often toilet in the same area.”
A good scrub of the part of the carpet that the pup's using with a solution of biological washing powder should deal with this.
OP, you'll notice as well that before your dog toilets, he'll start sniffing at the ground. That should be your cue to shift him to his designated toilet area straight away.
Good luck