puppy growling and biting

Question
Dear Anne,
I have a 6 month old terrier and poodle mix. He has been described as "submissive" around other dogs and is, by and large, an affectionate, playful and high-energy pet.
However, if he grabs something he shouldn't (socks, tissues, etc.) he refuses to "drop it" on command and will growl furiously and even bite when I try to take it away. Seriously, sometimes he doesn't even seem like the same dog. This can happen when he is chewing on a piece of rawhide or treat bone also, and though his tail is wagging, he will not hesitate to use his teeth. I have tried everything - playing "trade," holding him by the scruff of his neck, punishing him by putting him in his crate, and he immediately reverts back to his normal, loving temperament, but it doesn't prevent it from happening again.
He is enrolled in obedience class, and will be neutered in a few weeks, but I'm afraid this behavior might get worse. Any suggestions? I would appreciate any help, since this is my first dog and, much like a first parent, I'm afraid I might be doing everything wrong.

Answer
I'm surprised your trainer didn't address this.  Anyway, good book for you is Jean Donaldson's "Mine! A Guide to Resource Guarding in Dogs".  Do not take things from his mouth - only makes him want it more.  And, don't chase - makes him think his treasure is even more valuable.  For now, do not give him "special" stuff like rawhide to have on his own.  Instead, you hold one end and let him chew on the other.  Have some really good stuff handy, such as roast beef or liver, to "trade" back and forth for the object (you will still hold one end the whole time - and don't try to trade a Milk bone for rawhide - use something he never gets any other time.  Tripe, liverwurst, salmon brownies...).  As you feed the roast beef, say "trade", and as you give the rawhide back, say "take it".  Keeping one end of the object teaches the dog that YOU control the resources (therefore you are in charge, not him).  This is not a behavior you can punish away - and you can make him really aggressive by doing that.  Get the book, and never have him, a child, and food in the same room.