Does the radiator cooling fan run when the engine heats up?
You did not mention anything about your mechanic testing and/or eliminating the electrically-powered radiator fan as a cause of the problem.
At this point, due to the repeated overheating incidents, it is possible that a head gasket has been breached, or that other engine damage has taken place. I recommend that your mechanic perform a compression test and a cylinder leak-down test to assess whether there is a head gasket-related problem.