Not sure what to do....

So my fiance and I just moved into a new duplex with our son, we have a baby on the way and everything was great... we have been arguing alot lately so the past week it's been nice that we haven't argued, at the beginning of july we got into a bad fight, about his ex..she keeps blowing up his phone and instead of blocking her he makes up excuses on why he cant.. they were together for 6 years almost and they lost a child when they were young but that was at the beginning of their relationship.. she has crossed so many boundaries and he is just not getting why I'm so upset with him respite doing to her and then meeting up with her a couple times and had no intention of n telling me till i found out and flipped out. Well I thought it was all done and over with till I found out he was gonna meet up with her again but only for 5 mins to go to their sons grave, he again wasn't going to tell me and he lied on when the last time he talked to her was.. I'm debating on if I should tell him I know he lied or just let it go? I found out when I went through his phone