If you are going to run a length of ethernet cable to your son's room anyway, why would he connect "wirelessly" instead of with the ethernet cable?
Should you really want to install a second router then, provided your son doesn't need to be in the same Subnet (IP address range) as you for File & Printer sharing with your PC, you should install a "Cable/DSL" router, i.e. one that *DOESN'T* have an ADSL modem built in. You then connect its WAN port to one of your router's LAN ports. The second router will get its WAN IP address in the 192.168.0.x Subnet from your router and it, in turn, needs to allocate an address in a different Subnet (such as 192.168.1.x) to your son's PC.