If you are locking the doggie door so he won't go outside to do his business, your dog may be confused about where he should go. Had you previously been keeping the door unlocked so he had access whenever he needed?
Another thing to consider is what you are using to clean the urine under the table. Getting a special cleaner that removes the enzimes works better than using a typical house cleaner or sanitizer. A dog urinates where it smells urine--and his sense of smell is so much stronger than a human's that he may be picking up that odor.
Finally, if he has already gotten into the habit of using that area as a toilet, you may find it is a difficult habit to break. If you have no trouble with him having accidents in other parts of the house, you may need to block the area off so he has no access until he is out of that habit. This could take a month of keeping him out of the dining area.
Good luck. Housetraining difficulties provide a special kind of frustration!