He's young. Keep working with him and he'll grow out of it. I'm sure you've tried the usual list of recommendations: * divert his attention to something that he can chew on - give him a chew toy * firmly say "no" and stop playing with him (give the consequence to his action) * yelp really loud as if it really hurt you (this worked with my little biter)
Just keep at it. You'll get through it. You'll see that they bite less and less until they stop altogether.
I am sure your dog will grow out of it. My boxer is now 7 months old and has grown out of her biting, which was her way of playing when she was a puppy, it was her way of exploring. What we did to help is everytime she bit to hard we would say ouch and she learned quickly that it was unaceptable for her to bite that hard. There was a while I thought she would not grow out of it, but once her baby teeth were gone she did lots better.. We also let her play with other dogs because I have heard the other dogs will teach the younger dogs what unaceptable biting is too.