I have a question for you, how does he behave at home? How does he behave when you discipline him and how do you handle discipline? Are there consequences for ill behavior at home and what are they?
My son is only 5 but I think he was on a similar path. He's been mama's little angel and mama's boy and he used to listen perfectly to me. He was seriously ill as a baby, we almost lost him a couple times, so he had been overly spoiled since birth due to that. I litterally gave him whatever he wanted and didn't implement discipline or consequences.
By pre-school we had a few problems of beligerent behavior refusal to share, tantrums and hitting if someone else had a turn first, telling the teacher "no" disrespecting the teacher, just all around acting out.
I had to "ground" him for a week before I saw any results in changing this. No TV, No playdates, No desserts, No computer games, No Tae-kwan-do, No extra anything at all. But there was lots of talking to him during that week about why it was happening and how we were going to change it all. It took a HUGE effort on my part to treat him differently than his sister that week and give her all the usuals and not him!!!
They have to experience as sense of loss and realize all the things they have to be grateful for in life including a great mom who sacrifices a lot for her kids. Do you think your son is grateful for all he has? Maybe it's time for a reality check. I think he's acting out to see how far he can push everyone and get away with it.
This is a good time to assess and scan his environment and take a good long hard look at how the household is run, and operated and could anything in his immediate environment contribute to this?? It's hard to be really open about it in your mind, don't exclude yourself or his father if the father is in your lives. Also try to watch supernanny, she has wonderful ideas for ill-behavior and great ideas for rewards systems that can work wonders. I would put a rewards system together for him and with him during the week of grounding if you choose to do so. Make a chart or calendar for every day he behaves at school he gets stickers. If he can complete 3-4 days, he earns a small reward.
If those efforts fail, I would take him for a neurological pediatric assessment and for therapy to see if an outside party can help.
Don't get discouraged if you see him improve and then go back to acting out, it can takes months to get it into a "normal" routine of behavior. He will have relapses after improvement. Mine does every few months and has to go through gentle reminders by enduring consequenes of losing privledges.
Don't give up! You can do this!!! Good Luck!! God Bless!!