Box Turtle live outside?

I have a box turtle that I've had for over 2 years, who I believe was wild before he was captured and sold to us. He doesn't seem happy inside and spends almost all of his time hiding. He does love crawling around outside, though. I was considering moving him to the back yard which is much larger and fenced in. We would still provide food, water and shelter for him, and I just think he would be much happier. Is there any thing we would need to know, or any danger to letting him live outside?

If you check online, you will find nice designs for outdoor box turtle enclosures. You cannot simply let him live in the back yard--he will dig under the fence, and be lost. He would also be vulnerable to predators.
   
A proper outdoor enclosure will keep your turtle safe from raccoons and other predators, allow access to shade and sun, and fresh foods. It's the healthiest way to keep a box turtle in captivity.