Translation:He is asking about the difference between boys and girls.
"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
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.