C.T.
I think you have to be very black and white with him. Tell him what is and is not acceptable and that from now on there will be consequences. Walking away is unacceptable. So is hiding under the table. Etc. Have a talk with his teacher too and if he pulls something at school he will also get consequences once he gets home. Tell him there will be no 2nd or 3rd chances from now on. There will be no reasoning or debating. Try to think about what he would REALLY hate taken away. For my son it is video games, riding his bike, or having a playdate. Maybe for your son it is no TV for the next 24 hrs, or no computer time. Whatever motivates him! Once he does the behavior then give him the consequence. It sucks to be the "meanie" all the time but our 7 year old really shaped up fast once we became very back and white about things. Both his dad and I were on the same page too. We had to come down hard and never slack off or let things slide. We were in a similar situation as you in that we had just had a baby. On the other hand we also started really praising him like crazy when he did something right. It seemed to work great for us. Good luck.