If he rejected something known by default of Islam, he is a disbeliever. If he doesn’t believe in the Hadeeths in the Bukhari and Muslim, he is not a believer as there are lots of things that are in the Sunnah and not in the Quran.

A Shia friend of mine does not believe that Prophet Muhammad (pbuh) was illiterate. He also doesn’t believe in any of the hadiths of Sahih Al Bukhari, Sahih Muslim and so on. Is he a disbeliever—will he have to abide in hell forever?