Yup, I agree to stop the fetch games and take him for walks. I would also get him checked out by a vet. I see a hitch in his stride and it looks to me to be his right rear. Either way, I would get him x rayed and checked out. I would also check his spine. If you take his right rear leg while he is standing and flip his paw over so it is on his "knuckles" what happens? Just grab the rear leg above the foot and flip his paw over so it rests on the "knuckles" and let go. See how long it takes to right itself? Do this for both rear legs, one at a time. Normally a dog will flip it over and right it's paw immediately.
I would also trim him down a little, 80 lbs is too heavy IMO at this age for him. Bigger is not better, heavier is not better. Knowledgeable people do not judge a dog or quality based on how big or how much a dog weighs. If he were my dog I'd take 5 lbs at least off of him, probably 10. His weight is not going to help his hind end issues. You should be able to easily see the last two ribs and easily feel each rib when you run your fingers down his side.
He is a beautiful dog. Best of luck with him.
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