Constructive dismissal is certainly a risk here. Take a look at this analysis HRI did on the very issue, which is based on cases dealing with whether forced relocation is constructive dismissal. https://hrinsider.ca/hr-legal-briefing-is-requiring-an-employee-to-relocate-constructive-dismissal/
IF–and I’m not advising that this is what you should do–you do decide to make him transfer, be sure to get him to put his resignation in writing if he refuses. The letter should indicate that he’s decided to resign and specify the reasons for the decision. The employee may still be able to sue you for constructive dismissal (see article above). But having a letter documenting the reasons: a. minimizes the risk of surprises later; and b. might help refute other constructive dismissal claims, i.e., complaints about other things you did to constructively dismiss you, in a subsequent lawsuit.