American Bulldogs

I have a 12 week old AB (male) that I got when he was 7 weeks old. We are attending puppy kindergarten classes and going lots of places together for socialization purposes. The biggest problem I'm having is his biting. he bites HARD. I've tried lots of tactics to curb this, but nothing seems to work. I've tried holding his muzzle closed and telling him "no bite". I've tried a shaker can. i've tried a water gun. I've also tried holding him by the scruff of the neck and shaking him a bit while telling him "no bite". NOTHING works. he is growing at an alarming pace so I really need to get this problem fixed pronto. any ideas?