I cosigned a contract for my son's car. he defaulted on the loan. refused to give me the car but I took over payment. How can I break the contract?

There are two obligations here, one you made to the lender and one you made to your son.  You are obligated to comply with your committment to the lender, which hasn't breached the contract.  Your son, however, has breached the contract, causing you damages in the form of payments you have to make to the lender; you can sue him for those damages.  Unless you had some sort of agreement with your son that he would transfer title to you if he defaulted, and can prove that agreement, I don't think that you can compel him to transfer the car to you.