Your poor pup has been wounded. Maybe he got into something while outside, maybe he stepped on something, or maybe he had a fight with another animal. Once you get past the initial shock and worry for your pup, you will need to clean his wound to ensure dirt, debris, and bacteria don't build up and fester causing an infection and additional pain. After cleaning your little guy’s wound, assess him for a possible trip to your veterinarian's office. Try to remain calm because he can sense your energy. It will help keep him calm as well.
If your pup has been injured, he is probably in some pain and potentially even scared. His adrenaline may be running high, and he could either be acting rather subdued or excited and anxious.