P.M.
Sounds like he really enjoys his time with you and his dad, and likes being with you. At 12 years, he might really be longing for more "bonding" time with one of the most significant men in his life. Perhaps a weekend just doesn't feel long enough for him. If his mom is open to it and it would work for you, you might ask if a few longer summer visits would be agreeable.
It's really hard for kids to identify their feelings. Rather than trying to draw feelings/information out of him, I suggest trying the approach described in How to Talk So Kids Will Listen, and Listen So Kids Will Talk. (Read part of this practical guide: http://www.amazon.com/How-Talk-Kids-Will-Listen/dp/038081...) The authors demonstrate exactly how to support children in navigating their own emotional issues, and participate in finding workable solutions.