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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"?
No good means something has no use or value, and has no potential of becoming good. Not good means something is bad or undesirable. Source: Dictionary.com
So "no good at English" means he is not good at English and has no potential of becoming good. "Not good at English" means he is simply not good at it.
It would be rude and unkind to describe someone as no good at a subject, unless you knew them very well. - @KateBunting
