Termination while on Medical Leave
Yes you can terminate John and to do so would not be a violation of the Human Rights Code.
It would only become a human rights issue if you made the decision to terminate John, in part because of his disability. You were not aware he had a disability and the work performance issues you note were not connected to his disability. The Human Rights Tribunal will look very carefully at when and why the decision was made however. Practically speaking, it would be best if you have something to show you made the decision to terminate prior to his medical leave – an e-mail or notes from a meeting perhaps.