CoachB25,
Have you tried the back foot against the fence drill with him yet? My worst case last year was not correcting because he was denying it, at least to himself. After watching others do this drill he attempted and could not even swing because the fence got in the way. He had to do this 4 or 5 times before he believed it. Then he worked on the other drills with a purpose and the back foot fence drill became his check to see improvement. He got to where he could hit wiffleballs from that position without contacting the fence once.
....that's right Lamber...wiffleballs....