If I were you, I would try implementing MAC Address filtering (basically you white list MAC Addresses that should be allowed to connect). This can be a pain when connecting new devices, but worth it from a security standpoint.
I would also change your SSID and disable broadcast. Reducing transmit power may also help. If you don't have enough coverage with the reduced power, you could also go with power line adapter and setup another AP in the weak area.
Oh and you can also try reducing your key regen time too. If you leave at the default 3600 seconds, then you're also giving him a longer time to try to breakin if he spoofs your mac.
Finally, you can always go with static IP's or if your router is capable, setup DHCP to assign specific IP's to specific MAC Addresses.
Hope this helps!