@Michcamb83,
For this to work he needs another job. Not simply change departments, he needs a new place of employment so there is no chance is seeing her. You both need to go to counseling (individually and together). Sometimes it takes trying different counselors until you find one that really works for you.
Another Seperation may be needed also. Yes, absence made his heart grow fonder but it seems like you gave in too quickly. You need to let him know if he isn't willing to cut all ties, change jobs, go to counseling,, and give you open access to his phone in computer, things will not work. He broke your trust (and not just one time) so he will have to work hard to restore it.
I would persaonally put a spy app on his phone. He's lied before so his promises don't mean crap. Even if you do it for 6 months or so you need to know all communication with her is over and he's not talking to anyone else. Trust can be rebuilt if the other person is willing to do his part (support you, answer questions, go to counseling, give you access to everything, change jobs, etc). In return you have to learn to not bring it up or throw it in his face if he's really making an effort. If he doesn't make the effort you need to be firm and walk away from this relationship. If he's not willing to do what it takes to earn your trust Back, he's not truly sorry and it will probably happen again.