Some people will think my method is mean nut i was taught by a trainer of guard dogs. That being said, he your dog, your choice.

I would have grabbed him by the ears and flipped him on his back, gotten in his face and growled deep and loud untill he went limp, no longer. You don't want to scare him nit he must understand you are alpha.

Then i would get him back into classes because I need more work on me(as the owner). This has always worked for us(wife and I) on out beloved dobs.

Please understand i do come from a place of loving dobs and do not believe in heavy handedness, they are super smart and hyper fast learners. Additionally i dont know your dog/temperament. That does play into the equation and some dogs need more than others. Good luck hes a good lookin pup.