R.Y.
He is probably angry or frustrated and doesn't have the words to express it or much impulse control yet. Little kids are still learning words and how to use them--when they are overwhelmed by their feelings they can't always verbalize it (boys especially aren't good talking about feelings). My son did a lot of hitting at that age and the first 6 months of preschool was rough (he also had a new sibling). It helps to verbalize for him "I see you are angry you can't do ___" then explain after time out that everyone gets angry but hitting is not an acceptable way to show it. Then give him things he can do (hit a pillow, stomp, etc). With my kid (one of the most aggressive in his preschool class) he did get over it and by 4.5 had way better social skills.