Kids don't just turn all of a sudden and shift behavior for no reason.
Something is going on, whether it be in school, bullying or maybe even the teacher singling him out. Something has changed and it would be extremely beneficial for you to find out and squelch it.
My guess is punishing him won't do anything to solve his or your problem. My guess is the poor performance in school is an attempt for attention or asking for help of some kind.
Start asking questions; who are his friends, does he have new friends? Is there a girl? Is he getting enough attention from the teacher at school? Talk to the principal and his previous teachers and then go talk to his teacher. Something is different and it should be pretty obvious.
You have to break this before it becomes habit b/c it sets up really really really bad future pattern of behavior.
If you aren't able to figure out what it is demand compliance in the form of homework monitoring. Get a list from the teacher and check it against what your kid shows you. Ask questions and receive proper answers from your child. Be persistent. Once he figures out that there's no way out except through it will go alot better.
disclaimer: i am not a parent, but i was a kid once and something similar happened to me in 3rd grade. It was the teacher. I really wish my parents had figured it out...