How to get custody of my American born son?

Your message is a bit confusing. Are you all currently living in Pakistan? If so, you need to confer with a family lawyer in the country you are living. I could  only answer your question in terms of Wisconsin law. Under Wisconsin law, the custody law favors both parents having significant and substantial periods of placement with the child. It is rare where the court would award primary placement to only one parent and make the other parent a visiting parent, like every other weekend. There is also a law that prohibits the removal of the child from the state. You cannot move out of state or more than 150 miles within the state from the other parent, without providing advance 60 days written notice of the intention to leave the state, and you must also provide notice to the other parent that they have the right to object within 15 days. If an objeciton is filed, you cannot move away without a court order. It is impossible for me to advise you on what the laws may be on divorce, custody or removal in the country you are currently residing in.