Three things: which entity employs him, which listed profession the firm believes the role falls under, and whether the firm will put both in a letter. TN requires Canadian or Mexican citizenship, prearranged professional work in a USMCA Appendix 2 profession, and satisfaction of that profession's own credential rule. Self-employment in the United States is not permitted.
The three questions that decide eligibility
Employer identity comes first because it is the one that quietly disqualifies people. If the arrangement is an invoice from his own company, he is not being employed by a United States employer and the application is aimed at the wrong category. Second, ask the firm to name the profession.
Land Surveyor is a listed entry, and its credential rule is a state or provincial licence or a baccalaureate or licenciatura degree, so an Ontario licensed surveyor with a science degree meets it twice over. What can still fail is the job itself: if the duties are crew logistics and equipment scheduling, the professional character of the work is open to challenge. Third, ask for the letter in draft.
Officers read the employer's description, not the applicant's summary of it, and a letter that avoids the profession's language forces an officer to guess. Ownership interests should be disclosed in that letter rather than discovered later.