@Authorian 

It is not possible to throtle a particular LAN port on the router.  You can create access restrictions and content filters using parental controls.  Locking his access to certain times and child level content may encourage him/her to pay their fair share.  It is setup based on MAC addresses so it can be bypassed if they know what they are doing unless you also setup access control and witlelist the known MAC addresses connecting to your network.  This is probably the best you will be able to do native to the router.