I am sorry to hear that your boy has undergone some sort of trauma.
I would tend to agree with you that he either got bit by something, or got his toe caught in a fence or board or something of that nature.
I think that the best thing to do is give him time. If he had any sort of infection associated with this nail, it may have caused a fever and therefore triggered some lethargy at home.
If he isn't eating or drinking well, and doesn't show improvement in the next 2-3 days, I would have him evaluated by your veterinarian to make sure that there isn't something affecting him systemically.
I hope this helps!
As always, if you have further questions or concerns, please don't hesitate to ask them using the "reply" function.