My impulse is that you've given great advice to him, and now your work is done on that subject. If your son was small and the other boy was big, or you truly had some concern of your son being hurt by the other boy, I might think you should do more, but since it doesn't sound like this is a concern, I think you just have to wait and let your DS decide if he wants to stand up for himself or not. The only other thing I would say is for you to be sure to start pointing out instances of people standing up for themselves (without directly linking it to your DS's situation), so your DS can see examples of it.
And from what I've understood of your post, I think its a stretch to call what the other kid is doing 'bullying'. ETA: I read your OP as saying that one time the other boy threatened to punch your son. If it is something that happened frequently, I might feel differently.
How nice of your son to offer his bar on the day the other kid had no lunch.
When you start to cook, as when you begin to live, you think that the point is to improve the technique until you end up with something perfect, and that the reason you haven’t been able to break the cycle of desire and disillusion is that you haven’t yet mastered the rules. Then you grow up, and you learn that that’s the game.
Adam Gopnik, The New Yorker