Tell your son when you are ready or when your son starts asking. He is 2 and him starting school and you about to have a baby has nothing to do with a new man being introduced to his life, IN FACT, he is so young...he wouldn't even remember his dad NOT being his father.
This will only help him later to not have to deal with drama from all aspects of a seperated home.
He can call both of the men Dad, he will decide which one he feels is more his father as his relationships develop with both of the men.
Sometimes you need to stop controling a situation in order to gain control of a situation.
Your son has every right to know who his father is and let him spend time with him...Take your resentment out of the equation.
If your son is not in danger by being with his dad, then you should do the right thing.
Point is he will be loved, by everyone, and that's all he needs to grow and be a wonderful person.