How to hold back or let out anger?

ok so i am 16 and my step dad has GAD (Generalized Anxiety Disorder)a href="http://en.wikipedia.org/wiki/Generalized_anxiety_disorder." rel="nofollow"http://en.wikipedia.org/wiki/Generalized…/a He is 50 years of age. Today he was arguing with my mum over how Ethan ( his 3 year old son and my half brother) keeps doing the opposite of everything he is told ( he is new to being a dad, this was his first kid). He was really shouting in my mums face and i came over and told him to calm down.I said all 3 year olds are like that then he kept telling me to "butt out". He looked ridiculous and was going all mental, he dosent believe that he is the one in the wrong and the one that is over reacting. We have tried many times to get him to go see a doctor and we all tried counselling once but of course he was the one who said it wasn't working and all ( a way to get out of it cause he was beginning to realize that he was the one in the wrong). Anyway he started making crazy comments like" oh and you know what your talking about, your a 16 year old doctor" then he started laughing and all. I then said to him that he needs serious help and that he was ill. The he preceded to say that i will need serious help if i didn't stop talking, i think this was a threat. Anyway down to the point when he said that i just felt adrenaline rush through my body, it was like anger. I felt like i'd had enough and i was seconds away from punching him as hard as i could. the only thing that stopped me was that my mum was in between us. My mum keeps trying to tell me to ignore it but i have done enough of that and i feel this every time he tries to make nasty comments and all at me. he tries to threaten me. Anyway i'm 6ft and i am slightly more muscular than an average 16 year old cause i play a lot of sports. because of this i keep thinking i should just rattle him. I know its not the right thing to do though. especially because of my 2 wee half brothers, who are lovely. Does anyone know anything that can stop this anger and stop me thinking like this. What is the best thing to do?? Thanks for reading all this and taking time to answer my question, i appreciate it!