right,I like to say his English is poor

It depends on what you want to say. "He knows very little about English" means that his knowledge of English in general is not that great. For example, I know very little about Hindi. I only know that it is written with an alphabet that is not familiar. I don't know anything else about HIndi. I do not know any words or grammar in Hindi. "He knows very little English" means that he knows about English, but his knowledge of vocabulary, grammar, usage, etc. is not great, but he does have some knowledge of those topics. Hope this helps!

That's grammatically correct. Depending on the context, "She knows very little English," might be better.