Karelm you cannot simply output into 5:1!
In theory you could bounce a stereo mix through a Dolby Pro Logic 2 plug in or something similar to translate in 5:1 but this wouldnt be a true 5:1 mix and then not from Sonar without some serious hardware box in your studio....
You have to mix in 5:1 on a 5:1 system and then bounce down into 6 tracks (Front Left/Right/Centre/Rear Left/Rear Right and LFE).
Your Director is asking too much from you.
Does he even know his deliverables anyway? How is he going to utilise 5:1 sound on his final project? Is it for BR? Public screening (HDCAMSR/DCP tape?)......
How do I know this? I speak from experience......
Do your mix in stereo and let him worry about the rest.....