why not put a QoS rule in place, for that particular device (use either IP address or MAC address), and give him the priority 'low' ?
That allows him during 'low activity' of others to get full bandwidth, and when others are on the same router / network, it will give priority to other traffic, over his connection.
It seems to be working at my house, where League of Legends and Skype users (i know who they are 🙂 ) have low priority, and my VPN connections to work have higher priority
