Following up with a potential supervisor I have been applying to join up for an MSc in Universities in Canada. One particular faculty member conditionally expressed his interest in being my supervisor. The dept is considering applications and I have mailed them regarding the timeline of when a decision would be made with no response. Meanwhile, I mailed the potential supervisor with a change in circumstance and he said thank you for keeping in touch. It has been 15 days since then and I am wondering if I should mail him just to show that I am still interested in joining his lab. I am, however, unsure as to if I should mail him and if so, what do I say. Nothing new has happened to report and I don't know any projects that he is currently working on. Please advise.
Your potential supervisor, like pretty much everyone else, expects useful professional correspondence. By useful I mean that they expect to learn something new from it. If no event has taken place that would warrant a change of state, your potential supervisor has no reason to assume that your position has changed. Therefore, there is no need to send any email, that would confirm something they already know.

Two weeks is by no means a long time for an admission process status update, so, that needn't worry you. However, if the process would take a disproportionate amount of time (how much depends on local factors), I would consider emailing the professor again. But that is a change of the circumstances as explained above.

Another case in which I would consider contacting the person is a reminder. That is usually the case when you expect them something to do or if you got no response the first time. You state that you already communicated a change of state and that they acknowledged it by thanking you.

Bottom line, if you have nothing new to report, nothing to inquire and no need to remind the recipient of anything, there is no need to send an email.