"Lui ha non meno di cinque fratelli."
"He has no less than five brothers" is "correct" only colloquially. "Less" is used for amounts, "fewer" for numbers. One says "He has less money" but not "He has fewer money".
Correct, but "no less" can also be used as a sort of emphasis, eg. "He probably travelled First Class, no less!"
So this could also be phrased as "He has five brothers, no less" without being incorrect.
I like your point, although it changes the tone of the response from neutral to more surprised or disgusted or alarmed, etc. Perhaps someone who knows more Italian will be able to tell us how this more emphatic version would be expressed in Italian.