What to do if you know your boyfriend is lying?

I tend to agree with that.

At the end of the day, honesty is always the best policy. Trust in a relationship is so paramount. Once it's compromised, it's nearly impossible to re-establish.

@Maverick1962: i am glad you say nearly because i have seen trust be re-established but not over night - over a long period of very hard work and a lot of compromise from the party who broke the trust.