So I'm confused.
That could be because actually there are two issues here:
- grammatical case;
- singular or plural.
As for the first issue, "сколько" and "несколько" are numerals. Numerals govern the case of the word they're linked to, as described in your dictionary.
[...] and there's no such description for некоторый.
That's because "некоторый" is an adjective. It's a slave word. It doesn't command the case of the following words. Instead, it must agree with its master word, just like any other adjective.
So, you should decide which case you need for the noun and then use the same case for "некоторый".
Your example is somewhat reversed. If it's a verbatim copy from some test then the reasoning to solve it could be:
"некоторые" is a fixed word in this sentence. It's attached to the noun "man" which is animate. Therefore, "некоторые" is the nominative plural form (if the noun was inanimate then there would be ambiguity: either nominative plural or accusative plural). Next there's a choice of "мужчины/мужчин". "Некоторые" must agree with that noun, therefore the noun must be in the nominative plural form too. So it's "мужчины".
The second issue is singular vs plural after "некоторый". Sergey Slepov covered this issue in the comment pretty succinctly: grammarwise, both are possible. But the meaning will be different.
"Некоторый мужчина" might not be a very common collocation, but collocations with other singular nouns are well possible: "некоторое время назад", "в некотором смысле", "некоторая неуверенность", "с некоторой натяжкой".