what would you do if you found something out later about your boyfriend?

last year was the worst year for me and my bf, we argued a lot, things were so wrong and we lived separatlety.... but starting from this year everything is ok i guess, he asked me to move in with him and our son, we are trying to make everything possible to work, when there is something wrong we talk about it calmly and fix our problems, but last night i saw a text on his mum old mobile phone dated 7 november 2010 which was last year saying "i will make you come over mum is sleeping, i will call you ok" its obvious that he took his mum's phone and texted that person so that she come over to his house... i don't know who that person is and i don't know if she really show up... but i felt so bad and i wanted to tell him about the text but i thought about it first... it was in the past and now we are doing great.. what would you do guys?? would you tell him and leave him? would you forget about it and move because it's in the past? i don't know how to react... he cheated on me and it is now that i found out!!! so sad and i love him so much, i've already move in with him trying to fix our relationship and now i found out that last year that night he texted someone else to come over at his place... please i need serious advice... thank you