Well, frequency is normally pushed back to 7 days once they reach 100g, and yours is past that point. So you could start now.

As for when to give multiple mice: whenever the snake eats the one you give him and then actively looks around for more rather than heading for his hide to digest.
As far as I know (and I could be wrong) there really isn't a set size to give more prey items. You just have to watch and know your snake's signals that he's still hungry.