Grammatically speaking, It is more correct to say "his" than "their" (as their is meant only as a plural pronoun), and it is still considered by most english professors (male and female) to be less awkward to say he, him or his when referring to an unknown gender rather than he/she, him/her or his/hers. Additionally, the US is one of the very few cultures (tho not the only one) in the world where some people consider it sexist. It many cultures, it is a requirement that you use the masculine pronoun when referring to an unknown gender. Specifically speaking, in Spanish, every named object in existence (with a very few exceptions) has an assigned gender. When speaking about a person or persons of unknown gender, you ALWAYS say the masculine pronoun. When speaking about a group of people with even ONE male, even in a HUNDRED women, you ALWAYS say the masculine pronoun. Only if you are SURE it's a female, or SURE it's a group of SOLELY females, do you use the feminine pronoun. Just a little info for ya N

-Nicholas