Okey dokey,
Thats called prey-possessive, when he doesnt want to come to you immediately or wants to keep the item and chew on it... this is normal for young dogs, but you want to correct this as soon as possible.

Try throwing a ball and having a food treat in your other hand, when he gets the ball, call his attention toward the threat in your hand, your making a "trade" of his toy for another toy or food... See if this keeps his focus on the "game", not the toy. Meaning his treat for a good retrieve becomes another toss and another good retreive. Hope this helps.


EDIT: I wanted to add that you should really focus more on the OB, (obedience training) right now, and not on retrieveing so much. That will come with time. You need to have a solid OB to work from.