C.M.
My answer is the same. Invite his friends regarless of gender. My daughter always had and still has (she's 11) friends of both sexes. She's been invited to quite a number of boys' birthday parties where she is either the only girl from school or one of a very few. She's always gotten along great with boys (at least in part because she has a 14 year old brother). She frequently plays sports at recess with the boys because she enjoys it. They let her play because she's good. It's a great opportunity for kids to learn how to be friends with the opposite sex.