M.J.
We had a hitter at that age. Here's what we learned:
1. Don't force the "I'm sorry." This is viewed as a form of attention. You're focusing even more on him due to his bad action. It's counterintuitive, but you have to not force this.
2. Keep it simple. "No hitting" and remove him from the situation. Very clear: You hit = no attention. No talks afterward about what he did, how wrong it was, etc. Again, that's viewed as attention.
3. Try books. There are a number out there aimed at this group. Hands are not for Hitting is the first one coming to mind. They're really well done.
4. Talk later. Wait until a quiet time to talk about the hitting. Keep it short and mention hitting isn't ok. Talk about how he may feel when he hits and what he should do instead.
Good luck! And take a deep breath. I know it's a frustrating phase but it does end. :)
4.