The most improtant person in his world is a female.. he wants to be JUST like you!
give it time, explain that you can not have that operation done until you are a big person. Perhaps look at gender nuteral(sp) ideas with in your household rather then forcing male and female gender roles so then he will be ok to choose where he wants to go as he gets older with out feeling like he is upsetting you...

I know i always wanted to be a boy until about the age of 9.. it all sorted its self out in the wash.. Im still female.