I'm sorry for you and Ben. That was uncalled for and absolutely rotten. My son got picked on a lot when we were in London. I don't know why. It must have been that we had American accents. Or that he is terribly bright and verbal. Children went out of their way to shove him off of climbing frames, pour sand on his head, run straight into him rather than around him, trip him. Not one parent ever disciplined their child - and I saw them watching.
He went from outgoing, bubbly and friendly to terrified of children in the space of a year. Eventually the solution he found for himself was cross-dressing. When I asked him why, he said it was because "people treated him gently when they thought he was a girl." We spent a few months in frocks. He finally gained the courage to leave the frocks behind. (Especially after he figured out that people will treat a boy worse when they wear girls clothing.)
Anyway, long story short, he has figured out how to cope without dresses. But, the information he received on all ends was accurate. 1. Be who you are. 2. There are people who will treat you poorly when you are different. 3. It isn't right to treat others badly because they are different. 4. There are lots of ways to cope with it. Some are better than others. 5. It's best to just stay focused on what is working for you and ignore others who don't like it.
I know those lessons will be hard for Ben to grasp. But, by correcting the other child, you showed Ben their behaviour was incorrect. To cuddle him up and support him, shows him that he is loved for who he is. But, I wouldn't dwell on it or let it eat you up. Let it pass. (The frocks were hard...more so on DH...but we trusted our ability to support him and allow him to find his own way.) Just let your love be the anchor. He needs all of the information. he'll process it best he can. You'll be surprised how wise all children are when trusted, loved and supported.
Now, I interrupt bullying every time I encounter it. If the community doesn't let it go, it will stop. (But, I'm pretty brash and will walk a child over to their parent with a report.)