As I have seen done on here, try making a paper collar to put on him to see if he belongs to anyone with your phone number on it... I agree, that aggression is probably from him not being fixed.. Can you get him fixed and see if that calms him down?

I also agree that if he has owners, they are not doing right by him not having him fixed and leaving him out to roam and reproduce. He is one that needs to go "poof", even if he has owners.

This shelter.... it is not a kill shelter is it?