I'm slightly uncomfortable with the lack of an object pronoun here. Usually, you tell someone. Although "to tell" can be used on its own, it usually carries a slightly different meaning. So: "What can you tell about his face?" would mean something more like: "What can you conclude about it?" So you might answer: "That he's about 40, that he's probably European, that he might have had a broken nose one time", and things like that. You probably wouldn't list attributes like eye colour, size of nose and mouth etc. But if the question is: "What can you tell ME about his face", you'd probably interpret it more literally, and list eye colour, type of features, whether he has a beard, and so on. Although there is no "мне" in the original, I felt better translating it as: "What can you tell ME...", as this felt more natural, but was not accepted. By contrast, "What can you say about his face?" sounds perfectly natural, without having to add an artificial "me".
"Что вы можете сказать о него?" doesn't make sense. The correct sentence would be: "Что вы можете сказать о нём?"
Here is declension table - always helpful ;)
- Им. он (nominative)
- Р. его́, него́ (genitive)
- Д. ему́, нему́ (dative)
- В. его́, него́ (accusative)
- Тв. им, ним (instrumental)
- Пр. нём (prepositional)
The rule when to add н- is pretty simple: you just add it when он is being used after a preposition, e.g.
- без него - without him
- вместо него - instead of him
- для него - for him
- благодаря нему - thanks to him
- к нему - towards him
- за него - behind him (when movement is involved, e.g. "get behind him!")
- на него - onto him (when movement is involved)
- за ним - behind him (no movement, e.g. "the tree is behind him")
- над ним - above him
- с ним - with him
- о нём - about him
- по нём - after him
You may notice that the prepositional case doesn't have a form without the н- prefix. It is for a reason - you cannot use a prepositional case without a preposition :)