Ask your groomer or vet to check it's glands, if they are full that could be a good reason for the symptoms you mention.
Is it very cold where you are? My brother's dog loves to chase sticks thrown into the river. One time when it was very cold she ended up with a tail that hurt and she could not wag for over a week. No more swimming unless it's a very nice day.
What kind of dog is your puppy and what size? Do the other's wrestle with him? Are they much larger? Have you seen him get hurt by the other dogs before?
Anything else? Is he limping? Has he pooped today? How bout yesterday? What did it look like? More info. would be helpful.....
No, I don't think his tail is broke.........Karla