How to cut round hole in aluminum soffit after installation
This old house has an excellent video on why the soffit location is bad, and how to vent through a common shingle roof.

https://www.youtube.com/watch?v=PqrZWd_CQIE

How to Vent a Bath Fan Through the Roof, August 2014.  The homeowner installed the same way your contractor did, and in just one season there was mold under the roof.



Your contractor should pay for this: the soffit installation was incorrect and hazardous to your health.  You certainly don't want the vent inside the soffit, nor do you want it under the soffit.