I can't understand why people keep asking questions about Martin's books as if they are written in English, and that therefore the language should conform to English standards and rules. Martin writes his dialogue in pseudo-medieval English. Like Star War's Yoda's lines, he writes speech that is deliberately stylised to sound a little odd and quirky to native speakers.
In your example, 'proves' seems to be an abbreviated form of 'proves himself to be'.
So, yes, you could write that if you were trying to write in a slightly quirky way, and it suited the style of what you were writing. Otherwise, you should stick to normal English.