self defense is always a full defense, but generally what is called an affirmitive defense... where you must first admit to the crime and then show why it was permissible to commit this crime, if yorue unable to show... it creates a bit of difficulty in the case.
I would highly suggest that yo uspeak to his attorney and pre-trial discuss all of the potential ways that this situation can be shown to the DA, whether it be through the presentation of polygraph evidence to the DA or through any other way...
make sure you communicate with his attorney, if you are unable to do so then you shoudl consider consulting with another attorney immediately on this case.
Self defense is always a defense, even for a juvenile. If your son's attorney can convince the prosecutor that he was acting in self defense, then the prosecutor has the ability to drop this charge. However, not all prosecutors will just dismiss the case and some may want some sort of deferred disposition. if your son refuses to plead, he will have the opportunity to convince a jury that he was acting in self defense and can be found not guilty.
Keep in mind that this is a loophole in Texas law: a 16 year old can be charged as an adult for a Class C offense and isn't entitled to the same protections that he or she would get if tried in juvenile court. If convicted, this disorderly conduct charge would stay on his record for the rest of his life.
You and your teenager should talk to a local criminal defense about this case. Most do free consultations, but keep in mind Texas does not provide court-appointed attorneys for Class C cases.
Disclaimer: This answer is provided as a public service and as a general response to a general question, it is not meant, and should not be relied upon as specific legal advice, nor does it create an attorney-client relationship.