I think you have done the right thing so far. I think you have probably taken it far enough. Everyone who has been involved knows your stance including your son. I think taking it further would probably be too far. He might not have sounded sorry when he apologised but he was probably embarrassed. I think that if you take it further then he may feel humiliated and be less likely to feel sorry but more angry. Discussing it privately with him afterwards in a calm way (eg. ask if he is being bullied again, ask what he thinks he could have done better etc.) will help you to clarify whether he truly understands the consequences of what he has done. As a former victim he may have been seeking the 'powerful' feeling that bullies have. Maybe you could talk about other ways to empower him rather than this kind of behaviour.
I hope this helps. I am sure just like all kids he just needs a bit of guidance.