Congrats on your new family member! There's something so special about rescue animals... I really, truly think they come into your home knowing that you picked them out of everyone else there. It's amazing how you can just see that one face and know that's the one that's meant to come home with you!
Did they tell you anything about where he came from or what his background is? I adopted my last cat from the Humane Society, and she was a lot like your new guy sounds... just quiet, waiting for that person to come along... she'd been badly abused, and finally caught by someone eating out of a dumpster in a WalMart parking lot. She was SO scared when I brought her home, I didn't even see her for almost a week. It can be heartbreaking dealing with animals that haven't been allowed to blossom... I think the most important thing is to let the dog adjust to his new home on his schedule.
Here's a page from the Humane Society that has some good tips --
Bringing Your New Dog Home That's awesome that you're thinking about continuing to volunteer! Our SPCA makes it easy! They have something similar where you can sponsor either a space in the shelter or sponsor an animal until they get adopted - I believe they call them Shelter Guardians. Makes it a little easier, since we know we can't bring them all home with us.
Expecting pictures of your new pup!