What is really going on here? Is the guy joining your clan, or is he joining your family?

If he's joining your clan, then sure go ahead and check with a higher authority. If he's joining your family, then you are the authority.

My suggestion would be to have a small ceremony/celebration before the wedding during which you present him with the tartan so that he can wear it during the ceremony with your blessing as the head of the family (without falsely representing himself prior to the actual union).