I am pretty sure that the doctor who said your son was suffering from "laziness" has not gotten to the root of the problem at all--your son needs a full evaluation and a treatment plan. There are many good treatments available. I encourage you to find an expert at enuresis, the technical name for this disorder. A pediatric urologist who is part of a university medical center would be ideal. Even if you have to travel a distance to find such a specialist, it would be worth it. If you cannot find a specialist, take your son to another pediatrician experienced with enuresis, or ask another pediatrician for a referral to an expert on this condition.
In the meantime, whatever the specific cause of your son's problem--punishing him won't work (as you have observed). Your son cannot help it, and probably feels very ashamed. Punishment just makes him feel worse. Tell him that you now know that he cannot help it, that you don't want him to feel bad about himself, and that you are looking for a real expert to solve the problem. Be a team with your son, whose self-esteem depends a great deal on how this is handled.