From what you described it doesn’t sound like he is being OVERLY protective of the tennis ball. And you stated that he generally will listen to you. Both are positive signs for training. I started my dog on larger sized balls (like those rubber footballs, etc). She instictively loves to play, and would bring it back to me so I could throw it to her again. If she tried to run away, or make me chase after her, I turned my back and ignored her, at which time she would always bring the ball back so I would keep playing with her.
If she didnt initially give up the ball, I pulled it away from her, while firmly giving the command to “DROP”. After doing this enough times, she now automatically drops any ball at my feet when she retrieves it. Since you say that this only happens with tennis balls, why not try the larger balls, and hide the tennis balls for awhile? Then once he learns the “DROP” command, try to re-introduce the tennis balls.
Most importantly (as Cesar Milan always says) he has to know that YOU are the pack leader, and his job is to obey you. And that comes with time, repetition and firm commands. Hope this helps.