All of the above answers are fine. I am one of those to whom fdmaxey refers; I use "they". While "they" is plural, it is now commonly accepted to use when you have an undefined person being referred to, such as in your sentence.
In the end, the real answer is, do what you feel most comfortable with. This is an example of language taking a long time to catch up with culture. In the old days, men were deemed "more important" and they made the rules of language. As such, all situations which required an indefinite pronoun used "he" or "him" as appropriate. Today, our culture recognizes that the old rules were wrong, but our language has not created a widely accepted, non-gender pronoun.
He, she, he/she, she/he, s/he, he or she, she or he, or they are all individual answers to this issue. Use what you feel good using!