Hex sockets that size do not often strip out unless a worn tool is used; perhaps a new tool with sharp edges might get the bolt loose, after liberally spraying Kroil or PB Blaster onto the threaded end of the bolt after turning the bike upside down; give it some time to "creep" into the threads. After that, my suggestion would be to try pounding an oversized Torx wrench into the socket. Epoxying a sacrificial hex wrench into the stripped opening is another possible plan of attack.