To be sure, are you placing the pads in the same area? If so, how far away from your designated area is he going when he does it in the wrong place?
You're doing well in praising him when he uses the right area and treats can be good reinforcement too. Perhaps add an extra reward like play time or less confinement from your home for a given amount of time.
I am in the habit of not saying "no" when you catch them in the act but rather just fixing his choice by placing him on the pad. -Just because I don't want to add anxiety that the dog could possibly link to going poop or pee.
-These are just my .02 in an attempt to help. I'm not an expert and I'm a newbie to this forum.
Good luck! I hope to see pics of your pup!
