
Hi there
Although I don't have a giant breed (I have Golden Retrievers), they still need to have joints protected as they're growing. I would say no chasing of balls at all at 7 months. Jumping, twisting, leaping about in general is very bad!
I have a boy who was quite the nutty puppy, he's a big boy & at 6 months developed a limp which was diagnosed with OCD. Now, I will never know if it was a genetic problem or if, more likely, he caused excessive stress to his joints by throwing himself around so much. He never had a ball thrown for him, never went up the stairs, but I used to say he was the "boy who swings on chandaliers" or "jumped off wardrobes".
Even after surgery on an elbow and months of recuperation, he followed a Staffy when we were at the beach. I followed him onto the promenade, whereby the Staffy owner promptly threw a ball for his dog, who leapt over the wall, six foot drop the other side onto the beach... followed by my 8 month old puppy. It was the first day he was allowed off the lead for free running since the surgery and I walked all the way home with him in tears because I just couldn't believe the other owner.
We had to go back to square one with socialising and training after this, I was completley mortified, luckily it didn't have any long lasting effect.
He's now almost 5 and has just had to go back onto medication and supplements as he's starting to develop arthritis due to his OCD. It's something he'll have to have for good now, but at the moment we have it under control.
From my own experiences, I would never throw a ball for a puppy until probably a year old, and as far as your FIL is concerned, I would be tempted to shut the dog away when he visits. If your pup developed a problem after taking a fall as you've explained, you would find it very hard to forgive him, and I doubt he would take any blame anyway, just how some people are with dogs.
I hope you manage sort it out & not put your pup at any more risk of long term problems through someone else's thoughtlessness or lack of knowledge of large breeds.