Shouldn't it be "we were", not "we was"? Why was singular verb used here?
You are correct. The correct utterance is "we were like ... " and not "we was like ... " However, it is sometimes common to hear these types of grammar patterns in parts of the Southern United States, which is where this film is set. The main character of this movie is known to be a kind man but he has a low IQ, which is likely why he made grammar mistakes like this one.