Can I use AP approved after leaving from USA?

My son applied AP 3 months ago, but he could not wait, and left for China on Jun 20, yesterday, I got e-mail from USCIS, saying the AP was approved and sent to me. I'm wondering if I can mail the AP to my son so he can come back by AP istead of applying for H4 visa in China? Does any one has similar experience? I heard once you leave this country before AP approval, the AP will not be valid? Is this is ture? Thanks.