How come my friend doesn't get notified when someone messages him on facebook?

[Drizzy]

So sometimes when I try to talk to my friend on facebook when he's online, but sometimes he doesn't respond and it never says he's read my message. So later on he told me he doesn't get notified when someone messages him, when he's not on facebook. He doesn't get push notifications on his phone when he gets a message, nor he gets a badge icon over the messages when he's on facebook on his computer. He said he checked his notification settings and he said it's fine. Also he doesn't get those chat head pop ups when he uses his phone, he just gets a badge over the notification like on the earlier version of facebook for iphone. How can I help him? He also says he hardly checks his messages