Firstly, I would take this opportunity to praise my eldest repeatedly for doing the right thing, emphasising that 'big people look after little people', and that I was proud of him, and that this was responsible adult behaviour.
Secondly, in the conversation with the offending child, I would emphasise the same rule, that 'big people look after little people', and that I was disappointed in him and that he wasn't behaving like a big boy at all. I guess it's easier for me because I'm a man, and a simple frown or glare can sometimes carry with it a threat of violence. I also wouldn't take him off quietly to have the conversation. Ideally you would do it in front of his parents, since I wouldn't want them to think that you are spiriting their boy away to punish him. You actually want them to know what happened, how you feel about it,and what you said to their son.
If they object then that gives you the opportunity to *calmly* relate the facts of the situation and the rule of yours which you believe has been has been transgressed. Building up a history of letting them 'own' their kids transgressions sets the stage for your future actions if he does something like that again. If they never know about this incident, then any future escalation you engage in will likely seem excessive and a bolt from the blue for them.
Regarding what you do in the future if he tests you again, I think that the only thing which you can do is go through his parents, ultimately with something similar to 'I feel that your son bullies my children and I don't think they should have to put up with that.'. As long as you have reasonably and unemotionally explained to them the rules that have been broken on previous incidents, I think most reasonable parents will find your actions acceptable.