You can talk to his criminal defense lawyer about your idea, keeping in mind that you can end up both being convicted of possession even if you were the one who bought the drugs. If he knew they were in his truck, he's every bit as guilty of possession as you even if you were the only one smoking - and I doubt that the prosecutor is going to believe you were stashing drugs in his vehicle without his knowledge or awareness. (You even admit he knew - "We were just careless....")