there are numerous square wave generator topologies that can be used. You can google search to investigate them and pick one that makes the most sense for your application. Conversely, you can also just generate the square wave in simulink, and pass it to a controlled voltage source. To assist with the modelling, try using op amps and comparators from the simscape electrical library. The op amp in your current model is a simple op amp from the foundation library and does not have rails. Typically, simulaitng a square wave generator will require your components to have voltage rails to limit the output. (typically Vcc and -Vcc/gnd) An example of a higher fidelity op amp that you could use for this would be the finite gain op amp.