Can a kid with autism be really good in math and science. Or is that just grandparent impression?

Yes. Autism to me is a neurodevelopmental condition that manifests itself in markedly abnormal social interaction, communication ability and patterns of interest. I think autism is not an "intellectual deficit" but more of a low level "perceptual" disability. Yes, they can be good in math and science!
I agree with both . Till asked to understand algebraic concepts, they sure can. "data collecting, " related to over-developed rote memory & " pattern recognition, " related to strengths in non-verbal abilities, can make math computations a snap. Math applications, i.e., comprehending a word problem that requires language, active working memory, & deciding when to add, subtract, multiply or divide is not so easy.
Yes They Can. Dr sia's answer is correct, but it depends upon the severity of the autism. More severe cases are unable to display this talent.