Not by itself. Supervising non-professional staff is first-line supervision; L-1A needs management of a function or of subordinate supervisors or professionals, with real authority over staffing and policy.
Look for the layer between him and the technicians
The regulation defines a manager by what he primarily does, and it excludes first-line supervisors unless the people supervised are professionals. Mechanics and technicians in a dealership are skilled but generally not professionals in that sense, so a service manager whose day consists of assigning jobs and checking repairs is a supervisor. The case improves if the record shows a shop foreman or a parts lead between him and the floor, or if he manages an essential function such as warranty administration and service revenue with genuine discretion, even with few direct reports.
Ask the dealership for its organisational chart as it actually operates, the hiring decisions he has made, the budget he controls and the policies he sets. If those answers are thin, the L-1A route is weak and it is better to know before the petition fee is paid.