How is your dog now? Has the wound healed? It's not from him chewing, is it?
That sounds very strange. Did the vet say what might have caused it?
Can you remember any time recently when your dog was limping or appeared slightly lame for a day or two perhaps? It's odd that the hole is perfectly round. I wonder if it is some puncture wound maybe from falling against a metal object or similar?
It might be worth buying a small pot of medical grade Manuka Honey (make sre you ge a genuine one, packed in New Zealand. There are many fakes about!) and filling the hole with honey, then applying a dressing over the top, which would have to be bandaged on. (I don't know if you could make that stay on, or whereabouts on the leg the wound is, but if you could find a way to keep a dressing over the honey it would be good.)
Then replace the honey and dressing every single day until you hopefully start to see pink granulation tissue appear from the edges of the wound, which means healing has started.
It's worth a try.