In addition to securing your device with strong passwords, you should also examine any online email accounts you have. Be sure those passwords are changed as well, and check the forwarding section to be sure that he hasn't set up a forwarding address. This would circumvent any security on the device, as the breach would occur before the email hit your device. i.e. a forwarding address would essentially "CC" him on any email you get, as it is sent to your device.
Treat your password like your toothbrush. Don't let anybody else use it, and get a new one every six months.