My 2 y/o son has constant light bladder leakage. how do you treat this?

Hard to say: I'm not sure i understand what you mean. If he is toilet trained, but still having frequent accidents, that is quite common. If he is having a little urine leakage when he notices he needs to get to the bathroom, that is normal. If he has had good control and there is a sudden change, there may be an infection or irritation. An anatomical problem is rare, but possible. See your doctor.