That's what the guidelines say but you can send it before it expires. Send it by courier so you know it got there before his visit expires.
We were deterred from applying for an extension this way because of the expected 90+ processing times which would result in him overstaying his visa. BUT what you are saying is that if we apply, whether he will be rejected or not, during this period he will not be here illegally AND there will be no repercusions for overstaying??
Thanks for the advice... muchly appreciated.