I have never really had climbers, but we don't allow climbing on the couch full stop. There was a lil girl in the hospy recently when I was there who was five and had fallen over the back of the couch and broken her arm so badly she needed wires and an op.
do you have space for a plastic jungle gym type thing? I'd put one of them inside if you can with the foam matts underneath and redirect him to it everytime he climbs, tell him this is where we climb, not the couch. Good luck