You cannot regain your trust until he proves he deserves it. The bottom line is that he fucked up, so he has to fix it. It would be foolish for you to simply start trusting him again without him making some changes to show you that he merits trust.
Some simple examples are giving you full access to his email, facebook, cell phone, basically any method of communication that he has, for you to verify at your whim whether or not he's keeping secrets.
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker