Does he have a designated pee area in the house? We have it in our bathrooms since we live in a condo. I prop up water bottles (filled w/water) & wrap the bottles in wee wee pad. The boys use it as a marker, we've had zero accidents, but we have no carpet here (laminate). My boys naturally used the water bottle, I didn't even have to train them to use it.
I would remove all the plants too & anything else he used as a marker.
Spanking will not make him stop marking...if anything, it will make him scared of you. A firm 'No' & taking him to the designated pee area & saying the key words, 'pee pee' or 'poo poo' (whatever word you use) will suffice. You might have to do this a couple of x's for him to get it. They're very smart animals, they understand our words, tone & emotions. Please do not reprimand w/your hand.