Should I let my professor know that I cited a text written by him....that I pirated from online
It is incredibly uncommon to assume that someone bought a book just because it is cited. While in theory, the professor could ask this question, why would (s)he care? 
If you are geniuinely concerned about the question, perform an inter-library loan with your university library on the two books now, and should that question ideed pop up, reply that you performed an inter-library loan.
Use legal sources in the future. Libraries spend tons of money on buying literature, so use it. Most jurisdictions also allow limited copying of a book (e.g., the most relevant pages for your own work).