I do not think this is a protecting issue if you've only had him for a week.
Sounds like he has some separation anxiety as well.
So you've only had him a week? My uneducated opinion is that his world is completely different and he's trying to figure it all out, he isn't used to being on a leash and maybe that is causing some frustration when he meets another dog (another reason to get a trainer so they can actually see what is going...is it truly aggression? Or is it just leash frustration because he wants to go meet the other dog?).
Personally, I would take a step back and do a two week shut down with him.
And in the meantime, look for a trainer. If you can post your general location someone may be able to suggest a good trainer that uses positive based methods for you.