So what does the "U" on the identity document stand for?  Unknown?  Undefined?  Perhaps it stand for "Unicorn."  

Honestly, this is not progressive; its moving backwards.  People treat girls and boys differently because they ARE different, not because they're trying to impose gender stereotypes onto them.  The only reason we can speak of a non-binary gender identity at all is because there already are well-established social and behavioural differences, i.e "stereotypes" between males and females.  

This child will discover which camp it falls into soon enough.  Ongoing parental efforts to convince the child that it is "neither" male or female, or "both" male and female, will negatively impact their child's emerging sense of identity, and lead to emotional issues down the road.