Tennessee Law for Brandishing a Knife

I have a question for my father, who was just charged with aggravated assault. His girlfriend's son who had recently beaten her up (she failed to press charges or a restraining order against him, but he hurt her pretty bad regardless and there is a police report on the incident) confronted my father when my father entered their home (with approval and request by the homeowner - the mom) to bring her a wheelchair to the hospital where she currently is (unknown is her injuries at this moment are a result of her confrontation with her insane son). Once the kid got close and showed aggressive behavior, my father pulled out his knife and warned him not to come any closer. The kid called the police and now my father is out on bond.​I understand that this has nothing to do with a gun, but it is a concealed weapons question. From what I can put together myself as someone who doesn't live in TN, is that my father had more than enough reasonable justification to brandish his knife for self defense. Right now they're really pushing this charge hard, and my dad doesn't have the money to afford a good lawyer. I want to make sure he has the best defense possible, so I'm asking here to see what you folks have on your minds that I can lookup for him.​Thanks! via /r/CCW http://bit.ly/2Y7dPzf