The problem seems almost clear to me: Your 'mechanic' does not know what he is doing, get the car away from him. It is clear the issue is electrical, and he has no clue - same as 99% of the population of this world when it comes to electricity or electronics.

1) If the voltage at the clutch is not high enough to hold the clutch in, (which is what I get from your post), the issue is NOT the compressor, it's in the wiring or the climate control console.

2) Now, perhaps he is so incompetent, that he is seeing what is essentially NO voltage and assumes the voltage is 'weak' when, in fact, the controller module is not even trying to engage the clutch. This is a different matter altogether. The system will not engage the clutch if any of the following conditions exist:

The evaporator temperature sensor indicates an unusually low temperature. (This may be due to a bad sensor.)
The A/C pressure sensor indicates too high or two low refrigerant pressure in the system. (This can also be a bad sensor OR actual improper pressure.)*
The ECU thinks there is something wrong with the engine cooling system (the little "radiator" light will usually be on in this case, but not always.).

* If he replaced the compressor and did not charge the system properly, the clutch will not engage.