Another thing you could try over the summer, if he's up for it, is teaching him to use LaTeX or some other software for generating "pretty" math output. He probably wouldn't be able to use it for classwork, but it would allow him to turn in not only legible but lovely homework. (You might need to get permission from teachers to have the answers on a different page from the problems they hand out, but that wouldn't be a problem in my school. YMMV as always.)