How old is my pitbull puppy?

I adopted my blue pitbull from the humane society about a month ago. They told me he was 6 months and all of his papers say 6 months. However we think he might be younger. It’s bothering me that I don’t know his exact age- i would like to know what month to celebrate his birthday! Here is some info about him. If you know anything about pitbulls or just dogs/puppies in general please tell me what you think!

He’s 32 pounds. He DEFINITELY demonstrates puppy behavior. He is VERY playful and loves to chew on things (mostly our hands! he has not learned “NO BITES” yet… ouch). I don’t know anything about teething but a friend of mine looked in his mouth and she said he is teething judging by how the gums/teeth look. He has not lost any teeth yet that i’ve seen… but maybe that’s because he already lost them all…?? I read on google that puppies lose baby teeth around 4 months. Soo either he’s 6 months and already lost them, or he’s 4 months and about to start losing them. I have no clue! Also, he is pretty much fully potty trained. He does have an accident here and there, but for the most part he knows to go outside. Here are some pictures of him-

pic #1- http://tinypic.com/r/2u6ldtx/7
pic #2- http://tinypic.com/r/v612ee/7

in the second picture if you look closely at his teeth, it looks like one is just now breaking through the gum. i just noticed this. that pretty much confirms to me that he is currently teething. so about how old does that make him??

your feedback is greatly appreciated!! thanks in advance