["I don't feel good about myself right now. I took something from a boyfriend's house without him knowing.", 'Oops. Could you sneak it back in without him seeing you?', 'I thought about that but I was thinking I would be honest and tell him what I did and why. Do you think I should?']
Depends how close you are and how mad he'll be. Usually, I advocate honesty, but don't know enough about your situation to advise.
I took something from a boyfriend's house without him knowing. I think I should return it.