Hmmm - I don't have kids that big yet, so I may not be qualified to answer this. I've read some good stuff, Dr. Randy Cale, How to Behave so your Kids Will Too, and Children are from Heaven (all different authors taht I cant remember, but the Children are from Heaven is by the Men are from Mars Women are from Venus guy). These all suggest ignoring the behaviour you want to go away and really REALLY nurturing the behaviour you want to grow. So if you want him to put more time into studying (which is different from his school behaviour- I think that might be pretty normal for a kid his age - lots of transitions!) or into being helpful around the house or whatever, set up a system of rewards for that. It can be internal (doesn't have to be bribes), or could be external. Internal re: when you actually see him studying or putting away dishes or taking out the trash, MENTION it, ruffle his hair, sit down and read while he studys so he's not lonely. Whatever works in your family, it sounds like he's going through a typical 7th grade adjustment (a BIG year for adjustments - the expectations have just leaped!) in his own way, which is maybe more about pretending things don't hurt, or that he doesn't care about this or that, when really they DO hurt. He doesn't say it, but the change in friends, the harder work at school, even the expectations at home are leving him feeling like his childhood is slipping away and there's just not a darned thing he can do about it. Which is true...but he doens't have to feel like he's WEIRD or ABNORMAL for feeling this way. For external reward, you could leave him an extra something in his lunchbox, and say this is for being such a great member of the family!, or he can earn his privlidges back over time, or have a date night to the YMCA swimming or ice skating downtown or a movie with just dad or mom and dad or just mom, or a sleepover with a friend, or a special dinner cooked for him. All these would help him feel like 1) he's NORMAL 2) you LOVE him unconditionally ('cause guaranteed even if there's good behaviour, there's still going to be that negative stuff too) and 3) that he'll get through this phase of life and be OK.
I'm SURE it's way hard - I have no IDEA how I'll get through my daughter's adolescent years (already a rather trying child lol), but know that as his mama if you think he's out of character, you know him best. If his behaviour is not out of character FOR HIM, then allow him to grieve the passing of his childhood and move into the teenage years without feeling too freakish. It's already a pretty freaky experience!
It sounds like you're a terrific mama, and one who has a lot of experience. I'm sure there's nothing new here, but my rec would be feed the seeds of behaviour you like and starve the weeds - those you think are not worth his time (or yours). Good luck!!