So does Zeus. He was also adopted so I had also assumed that it was because of abuse. I'm glad to hear this may not be the case!
Zeus has been with us almost a year to the day (new year's eve!) and he's just now feeling pretty much 100% comfortable. It took him 6 months to pick up a toy and a couple more months to actually play for a bit. Now he will not even hesitate to join in the fun if he thinks you're willing to play.
Give it a year. That's what I've always said with various dogs and that's about how long it's always taken. In my experience, at least. Some are less, but the average is about a year. Don't get me wrong though - you'll definitely see changes in 2-3 months, but it's a whole 'nother dog in 12.
Thank you for taking this baby in!