I will have to take your word for it, and, again, I am not an expert in the education of low-IQ children, but what you are describing is way, way, way beyond the bounds of any kind of achievement I am familiar with.
[If I may be so bold, my best guess is that your brother probably had an IQ more like 95, rather than 80 - from what I understand, on average, with an IQ of 80, he'd be lucky to learn how to sign his name.]
He also goes to a school specifically for kids with disabilities taught by people who won’t accept “oh, they can’t learn” for an answer. Amazing what that can do. Tells me a lot of the problems in government schools is that they accept mediocrity.