I verified the settings on his device, but anytime he replies to an email or sends a new email, he does not have a copy of it in his sent items folder.
So you're sending an email from your BB device. Which folder do you want it to appear in? The sent folder on your BB device (message > menu > view folders > navigate to sent folder) or the Exhcange Server so you can see it in Outlook?
When I send an email from my BB, the sent email is saved both on my device and the exchange server. However, if I send an email from Outlook, the sent email is saved on the Exchange Server but that sent mail is not synched to my BB device.
You may also want to check the BES policy to see if there is anything preventing certain functions but the above mentioned method works fine for me.