How to deal with him?

With a boyfriend of 4 years and is all of a sudden a head? he used to be so sweet, never cuss, never calling me names, and always respectful and patient with me. Now he's an arrogant a$$ hole who has an additude problem with everything and he's always making dum remarks and faces if he doesn't like anything I say or do. He's 21 and he likes to drink and go to the bars with his guy friends and I'm 19 being a normal girlfriend to him and letting him have fun with his life. I tried talking to him but all he says I'm pissing him off right now and I'm so frustrated. What should I do? and What would you do?