You may want to try the /norestart command line option for msiexec just in case the MSI is ignoring the REBOOT parm. Below is a reference to the SEP Silent Uninstall Command Line
How to uninstall Symantec Endpoint Protection (SEP) client silently using the command line
Hope this helps
Thanks for your reply, I was able to get this to work by executing it as a background process, I found a reboot script that waits for a process to end before rebooting the server. This process seems to work OK.