Outlook unread messages appear read

In outlook goto the View Menu > Reading Pane and turn it off,
Sometimes the reading pane is situated at the right side of the screen and the users shift it more towards right making it completely invisible to us, but it is there, so select the inbox folder and turn off the reading pane.
Hope this helps.

the problem happens to all emails, so even if he was not logged in and recieved 10 emails, the next time he logs in they would ALL appear as read

Do you have installed any of those add ins?

       ICQExpress (COMAddin), installed as part of ICQ
       Business Contact Manager (BCM)
              Send to OneNote from Outlook PowerToy
       Outlock
              Outlook Backup Add-in       
       Sierra Wireless Outlook Integration - disable in the registry. It's listed        at HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Office\Outlook\Addins\SwiOutlookInt.Addin
       ONFOLIO
       Timeslips add-in
       Attensa for Outlook. Check for upgrade: newer versions not affected.        
       Cisco Meetingplace plug-in
       MindJet Mind        Manager 6. Disable the add-in in the registry - the key is        Mindjet.Mm6OutlookLinker.AddIn.2
       Google desktop "search your own computer", specifically        because it searches the mail
       ScanSoft PDF Converter Outlook Addin
       Norton AntiSpam Outlook Plugin
       Norton Internet Security
       AIM Sync
       LinkedIn Outlook Toolbar
       Salesforce
       Upshot        Outlook integration
       MS CRM 3.0 Outlook Add-in       
       Xobni - Upgrade to the newest        version of Xobni.
       Auto-Mate
       RingCentral Add-in
       DesktopSMS.Addin
       ABBYY PDF Addin
                               iTunes Outlook Addin
 

Maybe there are some rules as "mark as read"?

He has no rules running on his mailbox, and i am currently accessing his email from my own computer, that has no addons installed, and he only uses company computers to access his email, and users cannot install software themselves, so i am fairly certain that there are no addons involved

Suppose you select an email and right click on it and select Mark as Unread does the icon change to unread one or it remains the same.
Also if you leave it as unread and restart outlook does it stay that way or change to read status.

yes, if i mark them as unread the icon stays that way when i restart outlook

I would suggest that you try configuring this users outlook profile on some different pc and then check if it still remains the same, also check if the size of the users pst files. Also let me know if the default delivery is in mailbox or pst.

The problem persists regardless of which computer is used, and his mail gets delivered to an exchange mailbox

If the problem occurs for this user regardless of the system used then this is more towards the exchange server side than in outlook client.
Check the below link w.r.t your exchange server mailbox rules and see if you can find something that is not present for other users.
http://msdn.microsoft.com/en-us/library/aa142571(EXCHG.65).aspx

I have Outlook 2007.  I'm experiencing the same problem however, it seems the new messages arriving in my inbox are actually "unread". The problem is they're not marked in bolder text to indicate the message is unread.  To verify just right-click on the message and check to see if the option to "Mark is Read" is on the menu list.  It could be an Outlook bug since the navigation pane on the left displays distinctions between bold and regular text on various folders.