While you may have a basis for a lawsuit against the teacher, you need to consider what is the best, most effective, option. If the principal contacted you and you have corroboration from other students, I would suggest that you begin with a face-to-face meeting with the Principal and see how that goes. If you do not believe that the situation was taken seriously or handled appropriately, then you can assess your options.
I am very concerned about the hit to your son's chest since he is under the care of a pediatric cardiologist. Depending on what your son's medical condition is, he may need to have an individual health care plan on file with the school nurse so that all professionals who interact with him at school are aware of his condition.
You may wish to consult with an education attorney who represents parents prior to meeting with the school. The Council of Parent Attorneys and Advocates (COPAA) has a directory of professionals in your area that may be able to assist you.
This answer should not be considered legal advice. Each case is fact specific, and not enough information is available in this forum to provide any legal advice to you, just very general information. By answering this question, we are not entering into any lawyer-client relationship.