Pet sitting a JRT

I am a pet sitter, and am currently caring for a rescue JRT (along with another dog and three cats in the home, who are not the problem).

When I was here with the owner, the dog barked until she picked him up, then he was fine. But now that she's gone, Rocket has been much more aggressive.

He jumped up and bit me on the arm when I arrived this morning, and has been growling and charging at me nonstop. I'm not afraid of him, I just want him to calm down. Should I crate him? Ignore him? Win him over with treats?
Is this just a bad case of separation anxiety?

I thought that I had a lot of dog experience, but that was before I met Rocket!