He should be well on his way to "not" dong that by now.
I'd give a little scold first, then...
Blot or pick up the debris, move it to the Potty Pad, place it and praise him. He should associate the praise with the "right place". Do that again if necessary, and leave the debris there for a day or so, where he can definitely associate that area as being the right one.
I used a Paper Towel to either Blot up urine, or wrap solid waste in, where it just wouldn't be laying there exposed. You can place it near a wall where it won't be stepped on. He'll know what's in the Paper Towel.
There are also some "Scent Neutralizers" (spray) which you can use after cleaning those "wrong" areas.