You may want to try and base your decision on logic versus feelings.
It does not seem that this man is very stable. He had broken your heart already. Why take another chance? Just because he is having marital problems why should you be the one to whom he should go back for comfort? What he had already done should be a red flag and you would want to do what is best for you and your son. Your son would not need false hope that his dad and you can reunite and live happily ever after.