Translation:He is asking about the difference between boys and girls.
It'd be wrong.
You can't translate the prepositions using a dictionary. Every verb uses certain prepositions. These are NOT the dictionary equivalents of the preposition in a different language. You must learn the valence of each verb individually in each language. You cannot just try to translate the preposition separately as an independent word.
"ask for something" in the sense demand something is more like "žádat něco" . You can see that while the English verb uses the "for" preposition the Czech one uses a direct object. So it is obvious you can't translate the "for" here separately from the verb. A perfective equivalent is "požádat o něco" so another option. Notice it is accusative "o něco", NOT locative "o něčem".
That said I am not sure if "ask for" couldn't be accepted here, but I think it can't after reading the definitions in https://www.macmillandictionary.com/dictionary/british/ask-for