Best i can tell it's pronounced в when it's used in a genitive -его or -ого ending like ничего, твоего, кого, нового etc.
Words that are derived from old genitive forms follow this rule also. Сегодня = сего + дня ( this + day, both in genitive)
Words that happen to contain его and ого, but aren't derived from genitive are pronounced as they are spelled. Много (and other words derived from it like немного) and погода for example are pronounced as spelled.
I have yet to come across a word where его is pronounced as spelled, but I'm far from an expert.