I'd take him to the vet first, to rule out any physical problem.
Assuming there's no physical issues, it could be the other dog bullying him, perhaps a form of separation anxiety, attraction to any faint urine smell that might be left behind which could tempt him to pee there again, something upsetting him (new furniture or other environmental change?), etc.
It would be so much simpler if our furkids could talk! :wink: