Eastern Box Turtles are largely forest dwellers. The only time they would normally be seen out in the open is during egglaying season. They may be found in coastal areas where the soil has some sand content, but for the most part they live in deciduous or mixed deciduous/coniferous forests, so the soil has a lot of organic content from all of the accumulated leaves being naturally composted. They wouldn't be found on pure sand like you have in your box. They like to dig into the substrate, so he'd be a lot happier on a soil substrate. I'd just get a load of garden/potting soil and replace the sand or at least mix equal parts sand and soil. Some short, bushy shrubs would give him some natural hiding places too.