I'm pretty sure there isn't an option for this officially. However, I know a workaround. You can get it to push imap by using microsoft exchange (I did this on my old iPhone). All you need to do is enter in the proper server settings for your service (I'm guessing gmail) and let it sync. It should now be able to push imapAm I crazy or did Google still not enable push IMAP in the stock mail app?!
Could be a user error.