Rescue puppy of 6 months – scared of all noises outside

Hi! This is a combined vent but ALSO a real request for community advice on this issue. We've adopted a 6 month old puppy from Greece 1.5 weeks ago. He is amazing in all ways: so cozy, great with other dogs, nice and polite to people , smart and fun. BUT he is terrified of noises outside, and just being outside in general really: he does his needs and then wants to run inside.

We have made some progress in the last week (mostly from walking with other dogs, then he gets a lot more confident) but that progress all sets back to 0 if we are outside and a big truck goes by/someone slams a door when he's nearby/a plane flies above – then it doesn't matter if another dog is there or not, he panics and just wants to run off. Literally anything that makes a loud noise will have our little guy completely panicked.

We live in a city so we are not able to control his environment everytime we go out, even though our street is fairly calm (but not entirely noise free still). And I feel like I'm traumatizing him even more every time we are outside and there's a loud noise, and that makes me feel terrible thinking we won't ever have a safe and happy dog outside.

He's lived in a great rescue during the first 6 months of his life, so he hasn't been traumatized by sound, so I think it mostly is that he's never experienced loud noises before.

I've read a lot of advice online and we are in contact with a dog trainer who will help us in the coming weeks, so I know some classic things to do – but still would like some input from this great community.

Does anyone have any experience with this or a similar issue? What did you do? And how long did it take to get a secure and confident dog outside?
I just hope that he won't be like this forever…I see so much potential in him becoming a secure and happy dog but right now I struggle a bit with hope on this front as I've read so many "horror" stories of dogs never being able to get over there noise fears.

Thanks in advance.