Anna, congratulations on having begun your student teaching. I wish you a great learning experience!
My first reaction to your question is a deep concern about you having been put in the situation of subbing, In all of my supervisory experience with student teachers, one of the first things we make sure the student teacher knows is that they are not to sub in a classroom because they are not certified to do so. This is also made clear to the administrators when a student teacher is placed in their buildings, and student teachers are directed to always say they are not allowed to sub if asked. Perhaps this is a requirement specific to my state. I encourage you to talk with the student teaching supervisor assigned to you by your college or university about this, #1 to be sure you are not being put in a legally compromising position and #2 to be sure you are not being put in a position that is contrary to the requirements of your college/university. You can also find out your state's legal requirements by going to the website of the Board of Educational Examiners for your state. In my opinion, this is not an acceptable thing to ask of a student teacher. You are supposed to be learning and practicing under the supervision of a licensed educator. Period. I hope you will let me know what you find out about the requirements in your state. Please take responsibility for investigating this, even if the principal has told you it's okay, they do it all the time, it's an emergency and they need your help, etc. If something goes wrong and you are not legally allowed to be subbing, your career could be negatively impacted.
As far as the student refusing to work, I think you are wise to have followed your instinct that responding might have escalated the situation. Because you will be working with this student during your student teaching, you will have the opportunity to get to know him/her better and develop a relationship that allows the student to trust that you are interested in his well-being and want the best for him. You will also learn what triggers positive responses and how to best respond to his noncompliance. The issues of noncompliance and refusal to work have been discussed at length on this forum, and rather than repeat my advice or try to summarize the excellent advice of other experienced teachers who have responded, I encourage you to read through the other topics that have been posted. You will find a variety of interventions that might help you in working with this student, even under topics that don't appear at first to be specifically related to your question.
I wish you well as you continue your student teaching.