Hi Michelle,
My 6 yr old DD is the same....I have a really hard time dealing with all her arguing,backchatting and violence toward me and her 4 yr old sister...
I think consistency is the key with them when they are this age,and also you have to find things they really like or want to take away from them or ban them from doing as punishment....
I would start a star chart with him too maybe?? i did this with Jessica (til i got to slack to continue) and i really found it made a difference with her behaviour....
I love my girl,but seriously she pushes me to the edge sometimes too....
Have you tried sitting down and talking to him one on one about the way he is behaving and the way it is affecting you?? i found that made a little difference with jess....
And also i find sometimes with Jess that giving her extra attention,or maybe some time with just me doing something special works for them,because i get the feeling they feel like they are a bit left out when we have to pay so much attention to the other kids...
Sorry didnt mean to make this sound like a lecture,but i hope something i have said works for you!!!
Is he any better at the moment??