Do you think I should tell my son about my ex wife's infidelity
Why do you feel that there is a need for you to tell your son?
I think he needs to know the truth about his mother.
It sucks to be cheated on, I feel for you. However, telling your son may not be the best idea, as he may not understand what infidelity is yet, and may ask further questions that he would not understand until later in his life. How about you go ahead and try to make him visit your ex wife occasionally, and just try your best to answer his questions while protecting his feelings and understanding?
I'm going to tell him anyway.
I really think you should reconsider telling him. He's not really old enough to deal with that kind of information about his mother. This could harm their relationship.