I have a question ladies! David, my son's father (we are married, but in the process of divorce) is…

I have a question ladies! David, my son's father (we are married, but in the process of divorce) is in jail, and has been since I was 5 months pregnant. My son is now almost 7 months old. David's mother has repeatedly told him that he is not the father of my son, and at first he told her she was crazy & that he didn't doubt that my son was his. (There's no chance of my son being anyone else's, btw) But now, David is saying he wants a DNA test just to shut his mother up. He is in a state prison in Pennsylvania. Does anyone know how to go about getting a DNA test done with the other parent being in a state prison? I haven't, and never will, put him on child support nor will I ever collect alimony from him. I have no idea why his mother is being such a bitch! I have never gotten a penny from any of them for anything for my son. Sorry to rant, but this is something I have been dealing with for almost a year now! Any Advice??