Dane Adolescence

Question
I have a 100lb 6 month Great Dane puppy that I trained 2 months ago (to perfection I might add) who has become out of control.  He still knows his commands but he only obeys if there is a food treat.  He has begun nipping and humping and though he was neutered last week.  I kept him overnight and it was like a different dog.

Answer
I ran into the same problem with a Brittney Spaniel I recently trained. He started only obeying when there was food being offered. I ended up starting over using a different approach without food treats. He would still occaisionally get food treats,but there was never a guarantee so he wasn't so dependant on them.
Use the no free lunch policy(nothing in life for free) and make him work for everything,even potty breaks. Teach him that he has to sit and wait in order to be taken outside,he has to obey a command to eat,to be pet,etc. He'll be back to perfection in no time if you consistantly make him earn things he likes.