you posted the original concern 2 years ago. When did you find out for sure she was cheating? Has she been cheating this entire time, or only recently?
Sounds like you and she have major communication problems (among other things) trust issues.... those are defeating in any relationship.
What do you mean, she has your son? If she cheated on you and you feel like you should be part of his life you are entitled to that. Split custody, if you are innocent (a big IF since I don't know the story) then she has nothing to hold against you for wanting custody or partial custody of the child.
If you think she's unfit as a mother you could sue for full custody. Bottom line to remember in all of this. You must do what is best for the child involved. Regardless of your feelings for his mother.
And if you want to be a part of his life, the best thing you can do is be kind to his mother in front of him. Badmouthing the mother in front of the son serves no good purpose.