It's not a good sign that your mechanic appears lost already. Overheating can be a complex issue. If you have a quick, idle speed overheat this is generally an air circulation problem (since there is no air flowing, as the car is standing still, and thus requires cooling fans). So you have to check that your electric cooling fan is working first of all.
If the fan or fans are going full blast, and you are still overheating rapidly, you might have an air blockage in the cooling system (failure to bleed the system properly when adding coolant or replacing the thermostat).
If no blockages, you might pressurize the cooling system and check for a head gasket failure by removing the spark plugs, while the system is pressuized, and check for coolant on the plug tips.
You might also have a clogged radiator, which your mechanic should be able to diagnose by a flow-test, or even feeling the upper and lower radiator by hand.
Water pump is possible but generally if a water pump is failing, it is also making lots of noise and leaking badly.