It is perfectly lawful for the tenant to allow someone to move in with them without telling the landlord who that person is. You would only be able to gather information on the person if the tenant wants to add him to the tenancy agreement and have him be responsible to pay rent, at which point you could screen him as you would any new applicant to rent.

The law only considers a person to be an unauthorized occupant if they are left behind after the tenant has vacated the unit, which does not seem to be happening in your situation.

At the moment, you cannot insist on his information, as this can be seen as interfering with the tenant’s reasonable enjoyment, and may cause you to be subject to a Tenant’s Rights application at the Landlord and Tenant Board.