Great, thank you for this detailed response.
As he can bear weight on it and he does not seem to be in too much discomfort (happy to eat etc.), I would be most suspicious that this is a soft tissue injury.
Certainly, going over on the limb can cause quite a lot of tenderness, especially as he is quite a large and heavy breed.
The vast majority of young pups who suffer such an injury will recover quickly. Most will be 50% better by the next day and back to normal within a few days.
The best thing to do is to strictly rest him. He probably won't agree with me on this, but we really want him to relax and allow it to heal. Ideally, over the next few days he would only go outside to the toilet on a lead. We would avoid any running/jumping/slipping/stairs etc. If you can imagine, it is similar to a human with a sprain- the less it is used the quicker it will heal.
Try to keep his mind occupied with things like Kong puzzles and interactive toys so he does not become too frustrated at the lack of exercise.
Once the leg is back to normal, we can slowly (over several days) build back up to his normal exercise routine.
If the limp is not improving or if his limb is very swollen tomorrow (or if he is subdued), this would be cause for concern and would warrant investigation. The next step would normally be an X-ray, though with minor injuries we can often avoid this as it does require a sedation.
There are anti- inflammatory/pain relief medications that the vet can prescribe which can assist the healing and alleviate any pain, though these may not be needed if he is coping well and recovers quickly. Please DO NOT be tempted to give him any human medicine, which can be toxic to dogs.
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