Fluid and frequent feces are not a disease, but the symptoms of one. Therefore, it is important to eliminate the cause.
When a well-educated dog suddenly starts to dirty the , but is still very active, you should suspect neurosis. Has there been any change in his family? Is there, perhaps, a new baby in the family, or maybe a visitor has come to stay for a while? Have his sleeping place and/or eating times been changed? Has he been left alone for a long period?
As is natural, you can't get rid of the visitor to remedy the situation, but you can change some other things. What you basically need to do is pay more attention to your dog and only scold him when he has "pooped" in the wrong place.