Need advice please! !!

My 6 year old son was left in the room with my 7 week old baby for less than a minute and I heard the baby cry as if in pain. I ran to the room and found a red mark on his face and when I confronted my oldest he lied and said he didn't do anything. He finally confessed and said he hit his baby brother in the face and he doesn't know why. I have also caught him scratching the baby and I just don't know how to deal with this. He has bad behavior problems that we've been struggling with for several years, but I can't tolerate thud type of behavior. Any suggestions will be appreciated. Thanks