It's grammatically correct to say מִמֶּנּו for both "from him" as well as "from us", but to be clearer you should probably use a different word, such as מאיתנו as @airelibre said or מעצמנו ("from ourselves").
You're basically right, "from him" is מאיתו. In the bible, Noah sent the dove מאיתו, "from him", from his Ark.
However, "to hear from him" translates better in modern Hebrew as לשמוע ממנו, while "to hear from us" translates better as לשמוע מאיתנו.
(all of that, despite the fact that לשמוע מאיתו is also correct in more ancient writings, and לשמוע ממנו is a valid translation to "to hear from us", but will sound weird/unclear to modern speakers...)
So "hearing from" is simply a special case where I should deviate from the usual logic? :)