While I was recuperating from a car accident, my boyfriend was getting credit cards under my son's name. He was arrested and made arraingments to turn in his "partner" in exchange for a lighter punishment. Before he could be tried, he was killed in a motorcyle accident. How would I go about getting my boyfriend's motion of discovery and any other information concerning the details of his arrest etc...

If you know the case number you can go to the court house and get the existing records and see what they say. As far as the evidence against him etc you may not find much because they would set all that aside because of circumstances.
What exactly are you tryiing to find out?

Have you gone down to the DA's office and had a nice talk with them about this? If your son is over 18 he should go down himself as this is his credit, not yours, so he would be the one who needs to get to the bottom of this crime. If he is underage that's a different story where you would have some standing in knowing the information the cops/courts already have.