6th grade is a crappy year for boys. Not saying that to scare you or give him excuses....but telling you that 11 - 12 was about the worst year here for all 3 boys.
Their bodies are getting ready to grow, there is a lot of peer pressure to be "cool" in middle school. Their minds are a bit warped from all the changes and the stress. The social stuff becomes very important, where they never cared before.
One of mine became so concerned about the group he was or was not fitting into that his A's fell to C's in 6th grade. (I must tell you that if the group of kids he hangs out with do not value good grades and hard work...he is listening to their message.)
Since he has done the work and gotten the grades before, you know he can do it and is bright.
Rather than setting out for punishment, set up a plan for REWARD instead. When our Mr. Social Butterfly had grades fall apart, we took away video games, TV, etc....and he had to earn them back! But 3 of us sat down to devise the plan for school work and responsibilities. We had our son come up with ideas and write them down. For example, he felt it was very important to have about a half hour to blow off steam after school, so he could listen to music, have a snack, go outside. Then he came up with a study plan. After it was on paper, we agreed on how the video games, etc....could be earned back. Bringing all his grades back up to A's and B's got a special, pre-decided reward. And then all 3 of us signed it, like a contract, and put it on the frig.
Should he ever complain about studying or homework, remind him of the contract that HE helped create and show it to him if necessary.
There is a great set of books we used. Trevor Romain has written the BEST books for pre-teens!!! He has a whole series: How to do Homework Without Throwing up, & Cliques, Phonies and other Baloney, & a School Stress book are just 3 of them. There are on Amazon.com
All 3 boys read these---it's life lessons told to your kid by someone else. (After all, everyone else is smarter than you right now.) Excellent books! For Christmas gift or before!!! Every 11 or 12 year old should get them!
Oh, hang in there this year. Pray a lot! 7th grade gets much better once they have gotten the hang of middle school! And 8th grade is even better.