Hi. In concern of the point that you could not find the explanations for the preposition "про" from some dictionary, I would like to suggest you the multilingual online dictionary WordReference, available at: www.wordreference.com
From there, you may see the explanations regarding the preposition "про":
I expect that you are already aware of the fact that "про" takes on a substantive in accusative case, whereas "о" (as suggested by the other respondent Olga_L) requires a substantive in prepositional case. However, since in animated nouns/pronouns, their accusative declension is same as their genitive counterpart, and that for non-nominative cases of a third person singular/plural personal pronoun, an letter "н" is usually needed at its beginning when it is preceded by certain prepositions (for mainly pronunication purpose), and thus we have the declension "него́" in this case.
Good day. / хоро́шего дня.