Good child turned on me!

My 6 year old has always been a very polite, kind of shy kid. Never was in trouble at school. He was also an only child for the whole six years. We just had another baby in August. Well this year, he's been acting up at school, talking back, deliberately disobeying me and other adults. I've been trying to treat him the same as before the baby came but I'm TIRED! I'm a stay at home mom with an exclusively breast fed 5 week old. A first grader who's trippin! A home to take care of and a business to run! (I am married, my husband works 50+ hours a week). I feel alone and lost as to what to do about my son. Any suggestions as to how to curb this behavior before it gets out of hand?