My boyfriend and I just rescued this little guy from an animal shelter

He’s the shyest dog I’ve ever met. He was rescued from a home that had 20 dogs.

“One thing to know: many rescue dogs follow the “3-3-3 rule”:
First 3 days: overwhelmed, shut down, scared, or extra clingy.
First 3 weeks: starting to learn the routine.
First 3 months: personality really begins to emerge”

Any additional advice on helping him feeling safe? Besides being patient and not having loud noises.
Also we have not named him and not sure what breed. He’s probably a mix of a lot. Corgi maybe?