Thanks for answering my questions....I appreciate it.
Without being able to see your dog personally, it's hard to say what exactly could be going on...however, since we've ruled out trauma, problems with the rectum such as constipation or anal gland issues and problems with his belly, combined with the fact your dog is a power breed type of dog makes me wonder if perhaps your dog has 'limber tail'...or as it's called in Northern Minnesota, "Lab Tail"
We see many dogs with these flacid, painful tails and the majority of the time, we see it on Labs, although any dog with a thick, powerful tail can come down with it. I've seen it mostly in hunting dogs, but I've seen it in Chihuahuas and Bichons too.
You can find more information on the problem here:
In our practice, we generally treat with some kind of anti-inflammatory and rest. The tail generally heals itself within a couple weeks. Since it is a painful issue though, the best thing to do is to have him seen by a vet as soon as possible. If your regular vet is open tomorrow, then you can have them take a peek at them, otherwise, you may want to consider having him seen at an emergency vet tonight so they can get him started on some anti-inflammatories ad get him feeling better sooner.
I hope this helps!