I think that more than one thing is happening with the ball. I also think that he's keeping it from others, and that it's allowing him to use up his excess adrenaline, which is very important.
Yes, all dogs need some type of job, and specific types of dogs, such as retrievers, do best when the job matches up to what they have been bred to do.
At least he's finally mature. It does take a while with certain breeds!
I'm glad that he's gotten used to so many of the pets in the area!
Love All Life, Thank You For Posting! :)