I noticed the other day with a friend that our accounts were set to "not accepting friend requests." Therefore, it would just send him a message. I had added someone a long time ago, but it wasn't working when I tried this month, and this was the problem. Maybe this is a new feature. You could try asking him to turn it off (and checking your own profile) to see if that helps...
You may already have checked that, but just in case.. To do so, go to the "Your Profile" tab and drop down to "Account Details" ... then scroll to the bottom left to "Your Preferences" and click "change."
Hope that's helpful!