HELP! Agoraphobic dog resistant to training

Hi all. First time poster here.

I have a beautiful 9 month old mixed breed rescue dog that I rescued in July at 5 months old. When he first came home he had the usual shelter behaviors, timidity, fear and a need for safety and love.

He's lived with me now for 5 months, and he's turned into a gorgeous, loving family member. But he has some issues, and I've reached the point that I need some serious help.

Firstly, he has an older "brother", my nearly 5 year old Golden Retriever. My golden was trained with the positive reinforcement method, and he's a perfectly behaved maniac of pure golden energy. He knows when to potty, he requests outside time, and his recall is perfect.

Remus, however, has gone from timid to actively agoraphobic. I can't leash walk him at all – if he exits the house from the front door he has a panic attack. He can't be walked. He hides if anyone comes to the house. And, his potty training has completely reversed. He's now pottying upstairs.

I know he's capable of learning. He waits to eat, he sits for treats, he comes when called, he cuddles. He plays with his golden brother, he shares his toys when told.

So how on earth do I help him through this agoraphobia? And how do I get him back on track with the potty training?

For context, I believe it's loud noises outside that have caused this fear. We live in a neighborhood where you can hear motorbikes revving at 2am. There's frequently fireworks and other events.