I think he could excel in any subsport of distance running, but the way he runs hills and technical ground is out of this world, so it makes sense to me that he would focus on trails, mountains, and cross.
FWIW he was awarded USATF Athlete of the Week after this race:
http://www.usatf.org/News/Club-Cross-champion-Joseph-Gray-earns-USATF-s-Athl.aspx
I'm excited to see how he runs at BUPA and NACAC.