Comment
Great question.
There would seem to be three options:
(1) Use his and explain at the start of the document (if appropriate) that his means his/her.
(2) Use his/her (or his or her), but this is clumsy.
(3) Use their, but this is might appear grammatically unsound due to the change in number from singular to plural. (It is, however, an accepted convention.)
The choice is yours. All three are correct. Yes, even the last one (despite what some might tell you).
Thank you! :) 
We've added a page on his/her and their.