"This man sees his father."
Translation:Этот человек видит его папу.
Interesting question. I've barely scratched the surface, but from what i've learned so far, своего seems to be used in context of things very personal to the subject, like a dog, one's work, a book, etc. Perhaps it's not used with people - "his father" is just not the same as "his dog", because the latter implies ownership.
But I'd love to hear from a native speaker about this.
In my native Czech you always use the equivalent of своего (svého). The equivalent of его (jeho) would be inappropriate for either option. Looks like Russian is more benevolent but своего is certainly used even for people.
See also https://www.duolingo.com/comment/19968432/Usage-of-%D0%B5%D0%B3%D0%BE-%D0%B5%D1%91-versus-%D1%81%D0%B2%D0%BE%D0%B9-%D1%81%D0%B2%D0%BE%D1%8F and https://www.duolingo.com/comment/13931222/what-is-the-difference-between-%D0%B5%D0%B3%D0%BE-and-%D1%81%D0%B2%D0%BE%D0%B3%D0%BE .
Notice the example Я приду со своей женой there.
I disagree. Его is the dad of a third male person. By the way and not to put too fine a point on it, папа is dad. Father would be отец which would be отца in the accusative. I'm with the Czech in a previous comment. If you mean the человек"s dad, it should be свою папу or своего отца.
Just like in this sentence "THE man..." is clearly in reference to a specific male person. It is obviously not stating that the entire human race saw some man's father. This translation should be changed to "The person saw his father" or use Мужчина if that is truly the intended meaning.
(Edited, since it won't let me reply to @Alex162730 for some reason) Here's the thing... We're not saying the RUSSIAN sentence is wrong, we're saying the ENGLISH translation is wrong, at least as far as we have been taught so far.
We, native speakers of English, are saying that the only introduction that Duo has given us is мужчина=man and человек=person, even to the point of marking it incorrect to translate человек=man in previous lessons. So to give us an English sentence with "man", even including the masculine "his", the Russian sentence should have followed previous lessons unless further instruction/detail was added.
Maybe in Russian, человек can also be used more casually as "man" like "man" or "guy" or "dude" in English (not strictly masculine, just a general person) but we have not been given that info. We're not telling you that Russian is wrong, we're saying that Duo is being horribly inconsistent at best and deliberately imprecise at worst, neither of which are helpful in a beginner language lesson.