It does not take an IQ of 110 to “attempt” algebra. Basic algebra is certainly within reach of those with ~90 IQ. My youngest brother has mental retardation and an IQ of something probably around 80 and he could be taught basic algebra with some work (I’m talking about “find the unknown in the set of equations” algabra, not necessarily binomial theory)
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.]