Thanks everyone, for your kind words. We wanted to adopt Sunny from the beginning, but we were hesitant when we found out his history and the severity of his issues. The rescue actually suggested that we foster first, to make sure we could deal with his issues, which I think was very smart.
Our other dog, Maddie, is being a great role model for Sunny, and Sunny follows her around. Of course, he also witnessed Maddie chasing down, cornering, and almost catching a young rabbit yesterday in our yard (fortunately, she came to me when the bunny hid under an overturned wheelbarrow, but was FURIOUS with me when the bunny then escaped - she's such a terrier!). :R Sunny watches Maddie intently when we're loving on her, playing fetch with her, calling her to us, etc. He now doesn't have to be put on the tie-out outside (I still keep the leash on him, which he drags around "just in case"). He'll come back inside when called, following Maddie.
He'll learn to act like a dog yet!