House Trained Dog Peeing in House?

Our English Bulldog will be 4 years old this May. He was fully potty trained until last July 2015 when he started peeing on our bed, our couch, the recliner, his bed and blankets! He does it both when we're home and not home. We use to be able to leave him out when we were gone and now he's limited to two rooms when we leave.

I've taken him to the vet twice to get checked for any infections and everything came back normal. Everyone thinks it's a medical issue, but he's healthy! We let him out as much as we always have. I'm so sick of cleaning up pee and our furniture getting ruined, I don't know what else to do!