Wow that really is deep,Hard to say what it could be isnt it?I hope that Bon heels up fast,Does he seem to be botherd by it?The one thing i hated doing when Lacy had the punture wounds was keeping them open,The vet told me they heal inside out,Even with the antibiotics the healing process seemed forever.

Have you checked the other pups over?If your suspicion of a pellet gun or something else?