Post Surgery Uncontrollable Urine?

About 6 months ago my male Terrier got diagnosed with a bladder stone. We couldnt afford things at the time, so we had a charity vet do it for us. We took great care of him after surgery and if anything seemed wrong we went to the vet for check ups. He is doing great, very energetic. But I have noticed when laying down, he uncontrollably pees. I usually let him snuggle with me at night but now I wake up with urine stains all over my bed. He doesn't mean to do it. I take him on multiple daily walks. I don't know how to fix it other then doggie diapers. Is there any permanent way to fix it. (He doesn't seem to notice at all when he urinates. Just a puddles when he gets up)