I’m a guy in my mid-thirties. During family get-togethers, my step-sister’s husband (who also happens to be my father’s drug dealer) makes a point of ridiculing me in front of as many people as possible. He brings up embarrassing past events and generally does his best to humiliate and disgrace me.
If I ignore him, he gets increasingly more personal and aggressive. If I try to defend myself, my father immediately interrupts me with a litany of nonsense words or big fake belittling laughter before I can get a word out. Any time I’ve tried to talk to my dad about it, I’m “being to sensitive,” “imagining things,” or “can’t take a joke.”
I’m tired of being called out and then immediately shut down for others’ amusement.
LemonVibe 
Can you do without these family gatherings for a while? Will absence truly make the heart grow fonder? Depending on how much your father craves your company, I would try foregoing these family gatherings on the basis that you have better things to do i.e not be the source of family entertainment. Don’t let the “but he’s your dad” guilt you into feeling like you have to attend – your dad is being a jerk but also unnecessarily appeasing his drug dealer over you – his son! If there are calls or texts demanding why you are avoiding them, write a letter (seeing as you can’t get your message across) stating why. If your dad refuses to meet you half way after this gesture (or they treat the letter in an immature fashion), just leave them alone and be anywhere where they are not.
School finished a long time ago and you have to get on with the rest of your adult life.
I agree. I would find another holiday celebration. Just because you are related, doesn’t make them family. I would skip the letter however. He can call you and you can tell him then that you’ve had enough and are moving on since he honestly doesn’t care how you feel.
It will be hard for a little bit, but then it gets very easy. I had to cut my father off when I was in high school. He was a alcoholic, emotionally abusive and manipulative. Still tries to pull stunts to this day even though I haven’t willingly spoke to him since high school.