I know this might sound out of the blue, but is your son being bullied at school or in your community? I just ask because some children who begin to act in a hostile manner toward others are actually being treated that way by someone else, and that aggression may be a child's way of having control of a situation. It's worth asking a his teacher to be on the lookout at recess, when many classes are "turned loose" and minimally supervised.
Does he have an antagonistic or stressed relationship with a caregiver or another adult in his life?
The other thing I would strongly suggest is getting him into some counseling, now. Not when he starts to hurt or hit, but right away. Talk to your pediatrician and, if you feel comfortable, the school counselor, about referrals in your area. Many counselors offer a sliding scale fee, if need be. I often hear people complain of the price of counseling, but the price on your child to be suffering through life is much, much higher. Counselors can also help distinguish if there is an unknown factor in your son's acting out.
My best hopes for you and your family.