Thank you, Dala! The Kong is a godsend, and I love the idea of hiding the treats around the house! He really likes digging, so I'm thinking of building a little sandpit for him in the yard and hiding treats and chewtoys in it. Luckily, my landlords are incredible, and they've been helping me re-fit our yard to be more dog friendly for him. Pretty soon, I'm going to feel comfortable leaving him in the yard when I'm home.
Our mutty girl is also a rescue who was an adult dog when we got her. Our experience has been that she has mellowed out quite a bit since we have had her - it may take awhile for him to realize that you are his people and hopefully when he does he will chill out a bit. Our dog was fairly anxious at the beginning, especially the first few weeks/months. Take things slow with your new guy and he should relax some as he begins to trust you more.