Have to make one of the hardest decisions ever..... Do I stay with my fiance or leave for my son and I's sake. ....my fiance has cheated on me once again

....I honestly can't take the games , the lying, or the cheating anymore....I need to do what's best for my son and I.......talking to "fiancé " when he gets home from work. ...have photographic evidence that he's cheating. .....never thought I would hurt this bad....or make a decision that feels like one of the hardest ones ever.... :(