[Lillith]

At 20 months old, he's reaching maturity, and many of those herding breeds are known for reactivity and stranger danger issues. It may be he was genetically destined to become fear aggressive, it may be a direct result of the attack, or it may be a combination of both, but for this I would hire a professional behaviorist to get to the bottom of what's going on.

 

[Lillith]

Oh, forgot to add, you might also want to get a vet check just to clear anything medical, with a full blood panel. You should also ask your vet about doing a thyroid panel, as that can cause some pretty strange behavioral changes.