here are some signs to watch for with internal bleeding
and it says they seem fine at first
i hope you can get him to vet to be checked out
Symptoms of Internal Bleeding
Trauma
Accidents are the most common among all causes of internal bleeding in dogs. Often, the impact of an accident is so severe that the dog dies on the spot, due to heavy bleeding. However, sometimes, there may be slow internal bleeding which allows time to seek medical help. Most often, it's the liver and spleen which get traumatized due to accident. You should regularly check your dog for any symptom of internal bleeding, even after the treatment of trauma is successfully completed.