Unusual sexual behavior in my dog

Question
I just took on a permanent foster situation of an 8 yr old male Doberman. He has an extremely sweet disposition, but has one extremely unsettling trait.  Every day around the same time, 3 to 5 pm, he'll start humping repeatedly.  Me, the bed, my friends, the couch, it doesn't matter.  I tell him "No" in a loud voice and he immediately stops - for a minute.  That's not the puzzling part though.  I'll describe this as gracefully as I can.  The other day, when I told him "NO" for the 10th time, he took his penis in his mouth and frenziedly thrust into it.  He shook briefly, licked it twice, and then rolled over on his side and went to sleep.  

I am NOT making this up!  It was the most bizarre dog behavior I've ever seen.  

Have you ever heard of anything like this?  My vet hadn't and I've only found one person who ever heard of it and didn't have a clue what to do to stop it.

He's being treated for hypothyroidism and cardiomyopathy.  Could one of his medications make him do that?

Thanks for any help you might give.

Answer
Yes, I have heard of this behavior.  In fact, I once had an Italian Greyhound (rescue) who behaved in this manner!  Neutering did help, and he was much younger (only 7 months of age.)

I doubt the hypothyroidism and cardiomyopathy (both common in the Doberman, unfortunately) have anything to do with it.  I'm assuming you don't know the history of this dog (regarding when he was neutered) but obviously this is a highly self rewarding behavior (to put it mildly.)  The best way to redirect his attention (if you even can) is to train a behavior (over the course of the next three weeks, several times a day) using positive reinforcement (and you must make the reward a VERY GOOD one.)  Once the new behavior is acquired and the dog can perform it successfully ten out of ten times (100% of the time in your home, not outdoors), put him on a house tab (long lightweight nylon leash) in the middle of the day (about the time this unwanted behavior appears.)  The moment the dog begins to display the mounting behavior, pick up the leash, walk across the room and redirect him by requiring the trained behavior and jackpot reward when he performs (handful of small treats, diced cheese, chicken frank, etc.)  Then introduce a play object (large ball, basketball is good) and interact with the dog with the ball for a minute or two; then drop the leash and walk away.  Do this every single time the dog begins the unwanted mounting behavior.  There's no guaranty you will be able to totally extinguish this but you do NOT have to tolerate it (the humping in particular.)  If you are successful in redirecting his behavior, you may be able to truncate the full display (going from humping to, well...you know.)  IF for some reason the dog does NOT respond to this, LEAVE THE ROOM and close a door between you and the dog for one minute; reenter the room and request the trained behavior for reward, then resume normal activity.  Repeatedly remove YOURSELF behind a closed door and, upon reentering, engage him in his trained behavior; you may be able to assist this dog in developing some further cognitive skills.  It may be he was left alone a great deal of the time and found consolation in this sexual play.  AND you are most likely seeing ONE of the reasons the dog is in rescue!