A tick does not bite and then fall off that fast. A tick would be on him for many, MANY hours, feeding and engorging with blood, ballooning up to the size of a small pea before finally dropping off, leaving a small, hard, "knot" in the skin behind. That doesn't look like either a tick bite OR a black file bite to me. I'd wash it carefully, then keep an eye on it. If it gets bigger, call the vet.
BTW, dogs do not get at "bullseye rash" that humans get from Lyme, even if that was what it looked like.
BTW, dogs do not get at "bullseye rash" that humans get from Lyme, even if that was what it looked like.