You need to lay low and wait for him to cool off. No texting, no calling, just be reassured by what he said that he will be back.
My husband, while we were dating kept a secret Myspace that he didn't tell me about until I stumbled upon the knowledge because of something he let slip. He had 3 ex's on there that he had slept with who were still in contact with him. So, in some way I can relate to your bf about something being kept from your lover well into the relationship.
I love my husband dearly but no, I will never completely trust him again. You need to be prepared that this event may have sown a seed of mistrust that may not ever be completely uprooted. If he loves you he will cool off and come back, but he's going to wonder forever why you didn't say something when he came clean on his part. That is just the way it is.
But don't grovel too much. It won't help and will make the offense seem bigger than it is.