My step dad won't pay me my money?

I worked for my step dad for little while and he owes me a ton of money still, its not that he can't pay me, its just that he won't pay me... He bought my little brother (his son) a 100$ bike and he JUST got a bike too... I asked him for 15$ today because I want to go to the movies tonight, but he said "Sorry i'm short on cash" yet he and I both know damn well that hes not!! So any advice on how to get him to pay me my money? I'm 17 and he can afford it.