His issues sound similar to my DS (he is 4.5). He doesn't hit a lot but can get aggressive when he is angry and doesn't get his way. He will grab me, hit or kick at me, and will grab and bear hug his siblings when he is mad at them. He has sensory processing disorder and is on the autism spectrum (diagnosed PDD-NOS, ver high functioning).
I have found that getting angry, time-outs, and the like only fuel his anger and make it worse. Punishment doesn't seem to help. What usually works is a calm voice and and explanation fo why what he is doing is wrong. Also, sometimes he is just in sensory overlaod and needs me to hug him and hush at him to calm him down.
This may be what your little one needs. He is probably only doing this behavior as an attention seeking strategy. For kids like him and his history, an attention (even bad) is better than none. And he is so young that he doesn't know that he doesn't need to do that w/you. So, my advice would be to give him love when he does this stuff. I know, it sounds counter intuitive or counterproductive but children of neglect only want love and acting out is their way of trying to get it. You won't be letting things go or sending a message that aggressive behavior is ok. He will probably come to realize very soon that he doesn't need to do that anymore because you love him and his behavior will improve.
I hope this all makes sense. I can just relate to what you're going through and it is hard to deal with calmly. But it usually is the best approach.