According to my study "no good" means "not any good" or "not good at all".
So "He's no good at English" means "He's not any good at English" or "He's not good at English at all".
However, "no good" also means "not very useful or effective".
"He's no good at English" might mean he knows some English but his English is not so effective.
What is the difference between "He's not good at English" and "He's no good at English"?