You need what is called a WiFi Range Extender Router, which is nothing more than a repeater. You'll have to set the WiFi SSID and password into the range extender, and it will receive the weak signal and re-broadcast it for use with your other WiFi devices. You can keep it on all the time in his room as long as they allow him to do so.
If you're not tech savvy, it may be better to go with an easier range extender to setup such as the touchscreen
Securifi Almond. Otherwise, if you feel comfortable setting up routers, than something like this less expensive
Medialink should suffice.
I use WiFi repeaters all the time, by just re-flashing the firmware of a used $5. router. But this can be an intimidating process for those not familiar with router setup and configurations.