8 month old Newfie has begun to growl and bite while playing.

We have a very independently thinking Newfie male. He is sweet most of the time, definitely a puppy though. Lately we have noticed an increase in biting while playing. He will be playing just fine and then all of a sudden he just starts biting everything, hands, feet, clothes, etc. We immediately stop play, tell him no bite, but he won't stop biting. Most of the times my husband or myself must grab his collar and either rub his chest or belly to 'calm' him down. Sometimes that won't even help and we end up having to place him outside. Our children have been taught not to play rough and if we see them getting too crazy (usually my 7 yr old) we shut it down quick. I don't need a dog that size getting out of control What else can we do to stop this behavior?

We have also noticed him growling and nipping when he 'steals' things he isn't supposed to have. Mainly this happens with food items, but it has also happened with shoes or other items he is not supposed to have. This is a new behavior. He will usually stop when call his name firmly, however he will act like he is going to nip when the item in question is attempted to be removed again. Ideas on what to do?

We have finished his first round of training classes so he is familiar with certain words like leave it and what not. We have tried those and they don't work. Sometimes treats help to get the item away without continued nipping.

Is this a 'teenager' like phase he is going through? Should we be more concerned? We plan to continue training classes.