I pretty much sympathize with you. Betty is a 9+ year old Cardi. She will only let three specific dogs anywhere near her. No more doggy parks and great care when walking near other dogs. She was badly treated as a young dog and this may have something to do with her aggression toward other dogs. Corgis are herders. They are bred to contain and protect what they perceive as their "herd". I am not saying this is your problem but it may have some bearing. He has been with you for a long time and now there is an interloper in the house. He may feel it is his duty to protect you from this other dog. I am not in a position to diagnose long distance. I would suggest a doggy behaviorist. The humane society usually has a group of doggy behaviorists they can steer you to. 

Hey! We'd love to meet you and your corgi if you can make it out to the meet on the 4th of July