Is there any open source software we can use instead of MATLAB?

GNU OCTAVE (http://www.octave.org), which emulates the MATLAB programming language, has a control systems toolbox that is likely to have the level of tools required for CDS110a.

But some things are subtly (or overtly) different than in MATLAB, so no guarantees things will be easy. You would be forging your own path here, beware.

It doesn't appear that numpy or scipy (http://scipy.org) have control systems-specific tools yet.