I would DEFINATELY NOT smack your puppy like your trainer suggested or did! My God, how dare He smack your puppy, I'd have smacked him in the face if he'd done that to Jett
. Your hand is something your pat your dog with and give him hand signals with, you dont want him to see your hand as something that hurts him. That's just wrong on sooooo many levels. Your dog will never trust you if you smack it, not will it obey you, your dog will run from you. Smacking can also backfire by making the dog even more aggressive, and in fact it does not teach the dog what it should do instead of biting or mouthing (yes i know those baby teeth really hurt)
Keep going with NILF your boy will definately benefit and so will you.
Use your voice when correcting him, have a toy on hand and redirect his biting/mouthing to the toy. Dogs are very intelligent animals he will soon learn that biting and mouthing is not acceptable. I have had 1 whole week of no mouthing from Jett (she is now 6 months old). It does take time and persistence but please please do not hit your dog like your trainer suggested.
When he goes to bite or mouth tell him firmly NO. Use the same tone of voice everytime. aA soon as your dog does the right thing, pet him and say "good boy" he will respond to positive reinforcement. You have to be quick with the praise so that he will be able to associate what is going on.
I am not sure about using the crate as a Time Out as such, as you want your dog to like the crate and not see it as punishment....perhaps if your really have to put him in time out, toss a bone in there with him so he has something else to think about.
Good luck, its an awful stage of puppyhood but it is one that MANY of us have gone though/are going through.
Trina