It is a tough one. I usually start by asking if what the other child said was true. Virtually always it is not, and my child knows that. Which helps to build my child's confidence a little bit. And then we talk about possible reasons that the other child would say something like that. And then we talk about how the other child shouldn't say things like that because it hurts feelings. And finally, we talk about what my child could say the next time hurtful (and wrong) comments are said.
So, basically what you did, which is to acknowledge the hurt, talk about the ridiculousness of the comments (and here I like AvoCado's advice), and then come up with something empowering.
BTW: this kind of thing is definitely not limited to a situation like this, i.e., acceleration. Any time a kid is different in any way, kids will say hurtful things. The best thing you can do is to just be there to talk it through with him.
Although, if it happens again I would probably talk to the teacher to give her a head's up. You can attempt to have a first grader stand up for himself, but you can't expect it.
