Not on TD status, which permits study but not employment. Massage therapy is not on the USMCA list, so he has no TN of his own on that basis. He would need an independent route or must plan on not working while in TD status.
TD is presence without employment
The dependant status attached to TN carries no work authorization, and the rule does not bend for licensed health occupations or for self-employment from home. A registered massage therapist from Harrison Hot Springs has no listed profession to rely on: the list includes physiotherapists, occupational therapists and several other health occupations with their own criteria, but not massage therapy. If he holds a degree in a listed field, that changes the analysis and should be checked against that entry's criterion.
Otherwise the household must decide whether the general manager's salary supports both of them, or whether the move should wait until he has a route of his own. He may study in TD status, and children may attend school, but neither may work. One further caution: a spouse who arrives on TD and quietly takes clients puts his own status and, potentially, the principal's at risk, because unauthorized employment is a ground for refusing future admission.