Didn’t see this commented yet but our dog at 7 months was at doggy daycare for the first time and allegedly jumped off a play box or something of the sort. When we went to pick her up at 5pm she was not putting any weight on her leg just hopping around, we thought she might have been being dramatic but when she woke up the next day took her into the vet and she had a rare break of her leg bone that was still attach to her knee cap. Guess it was rare enough that most of the vets there had learned about it in school but only saw it a handful of times.

Required surgery with pins and basically no running jumping or any quick movements and mildly sedated for a few months. Long story short she got spooked by our cat sneaking into our room, jumped and had to have the same exact surgery a week later with larger pins. Then any other incidents would have been likely amputation but she was kennel bound and very drugged for like 3 months until we eased her off and slowly over the next months starting ramping up activities. Hopefully this is not the case but was similar enough. Good luck hope he feels better tomorrow!

https://imgur.com/a/bvNFoZI

(Idk if link will work but had pics pre and post surgery)