-его, -ого are widespread endings of adjectives.
Его is also form of pronoun Он.

You ALWAYS pronounce В instead of Г in these cases.

However!
Ого is interjection (English 'wow!') - Г is to be pronounced.
-ого-, -его- occur in the beginning/middle of word - Г is to be pronounced. Examples: Негодяй, Егор, Огонь, Погода, Прогоню.