On a DLink dir-615 router, how can i restrict network speed to a specific mac address?

I have my laptop, an xbox and my flatmates PC on my wireless network, and stream from my laptop to the xbox, but I always get slow net speed and network transfer rates because his pc is constantly munching up the bandwidth. Is there any way I can go on my router and limit his usage to 50%, or guarantee set speeds to my laptop and xbox?