I think you are definitely doing the right thing by restricting his furniture access, and enrolling him in classes. The training has really helped us with Jackie. She rarely "barks back" at us anymore, and I usually give her a short "time out" in the kitchen when she does. (This technique will probably not be as useful on a young puppy like Dudley yet.) Our trainers have said the same thing about "no" being overused. I would definitely follow Linda's advice.
All puppies are going to test their position in your "pack", just keep being firm and consistent. Remember, these dogs were designed to boss cows around, so they are going to have an opinion about almost everything!
Once he realizes that YOU are the boss (not him), things will get a lot easier. It takes awhile, so hang in there!
- Jessica