AB puppy biting

Question
I bought my american bulldog puppy at 6 weeks.  He is now 12 weeks old.  When we first get home from work he is so lovable, friendly as can be.  After a couple of minutes he gets agressive.  He starts to bite anything he can get his mouth onto. I have a lot of toys for him to play with and he will play with them at times.  But he will bite our hands, pant legs, and he won't let go.  He begins to growl.  I have bite marks on my hands from him.  I enrolled him in puppy kindergardern last week, she told me to grab the back of his neck or to hold his mouth shut untill he calms down, this seems to make him more pissed off at us that we have to put him in the kitchen.  Do i have a crazy dog or is there something we can do.  I honestly love my dog, but i have a 2 1/2 year old son that i don't want to get hurt.  What can i do????

Answer
First of all, you need a new trainer.

Now, this is normal pack behavior for a young male bulldog. Keep in mind that the only social reference he has is with this siblings, which includes, growling, biting and barking.
It's up to you to teach him what is socially acceptable behavior with humans.

You don't have a crazy dog but you do have a head strong puppy and this isn't necessarily a bad thing. If you can't divert his attention to one of his toys when he plays rough, then put him in his crate for a "time out" for 15 minutes. You can take him by the scruff of the neck and shake him HARD with a resounding "NO!" when he bites. It's ok if he complains. That means you got his attention.

This is a trick that will work but you have to be very serious about it. When he bites you hard, immediately take his paw and bite his toe until he cries. Then give him a big  growl and walk away, ignorning him. He'll get the point in short order.

Seriously though, you need to find a trainer who has experience with bulldogs because these dogs require good training and consistency in their lives. As for the 2 1/2 yr. old, just monitor his interaction with the puppy at all times until he's trustworthy.  This is as much for the child's benefit as the puppy's.