Attitudes are heart issues. We can't force a heart change, only a behavior change. My belief is that the heart issues are far more important than the behavior! So, you need to focus on getting to his heart. Making faces and groaning are all subtle signs of rebellion, no matter how common they are in pre-teens.
I would sit down with him and describe what he's doing and how it "looks" to you. ie that it looks like rebellion. Be very positive and say things like, "Now Mommy knows you do not want to be like that, right?" When he finally agrees with you that he does not want that attitude, discuss with him what will help him to change it. He may have an answer for you. Pray with him about his heart change. Let me caution you here, do not pray with him UNLESS he has told you he wants to change it. That is the key actually - to have him tell you he wants to change...because at his age, he's trying to find some kind of control. He knows he has to obey, but the faces are a way of exhibiting that he is still in control.
Addressing heart issues is an area that takes a lot of positive reinforcement and praise. Be all over him with praise when he does something without grumbling. In a gentle way remind him of the verse, "Do everything without complaining." The Hide 'em in your heart CD has the song on it. There is really so much to help affect heart change, that I fear this post will get far too long.
This is how I would handle grumbling. When I ask a child to do a task and they grumble or make faces, I would likely respond in a funny way rather than in an authoritative way. By smiling really big and saying something about smiling makes an unpleasant job a lot more fun, grumbling only makes it feel worse.
I would ask if he wants to do an experiment today. Assuming you get some kind of affirmative response - Tell him you're going to do an experiment about attitudes and behavior. Tell him that for a couple of hours when you ask him to do something, he is allowed to grumble and complain. BUT - he has to think about how it makes him feel inside. Then after a couple of hours talk to him about how he felt doing the jobs while grumbling. Then do a couple of hours where he is to respond in a positive upbeat way, such as saying, "I would be delighted to Mom!" when asked to do something. Then come together and ask him again how he felt inside when doing the jobs in a positive way.
The bottom line here is that he needs to see that his attitude plays a big role in how he feels about something, and that you can't force him to have a good attitude, but that it is really in his best interest to have one. Then sit down and work out a good solution for him to have a good attitude. Pray with him about God changing his attitude. Then, now this is really important. Ask him to go pray by himself and ask God how you can help him change his attitude. When he comes back with a solution, then you can just remind him that he has asked you to help him.
I know this was a long post - This topic is dear to my heart -One of the chapters in my book, I'm Going to be the Greatest Mom Ever...Even if it Kills Me! talks about child led behavior change. I'll tell you - with eight children, it has been extremely effective.
T. (too) Camp