S.F.
As a f/t stepmom to a now 8yo boy who was the SAME way, I feel your pain!! My stepson came to live with us around that exact age, for many reasons, that being one of them. I can't say what will and won't work w/ an agressive child - we STILL face issues.
I can say that I received some great advice/pointers from recommended books by his behavioral pediatrics office:
The Strong-Willed Child: Birth Through Adolescence (by James C. Dobson)
The Explosive Child: A New Approach for Understanding and Parenting Easily Frustrated, Chronically Inflexible Children by Ross W. Greene
Teaching Behavior to Infants and Toddlers: A Manual for Caregivers and Parents by Bernice Stewart and Julie S. Vargas
Positive Discipline for Preschoolers, Revised Second Edition: For Their Early Years - Raising Children Who Are Responsible, Respectful, and Resourceful by Jane Ed.D. Nelsen, Cheryl Erwin, Roslyn Ann Duffy, and Jane Nelsen
I know it seems like alot - but if you're like I was, you're at a point where any suggestions help - most of these authors have a degree in child development or psychology. They're well worth the read even if you only get one helpful tip out of it all, it's very nice to read that other people actually do walk in your shoes!!!