How can my 19 yr old son subpoena his ex girlfriend for a DNA test for their son?

In order to require an unwilling person to submit to a DNA test to determine the paternity of a child, there needs to be some sort of legal action pending. A paternity action filed by the mother, or a legitimation action filed by a putative father (like your son). In your son's case, he could file a legitimation action against the mother seeking to have the Superior Court declare him to be the legal father. The mother would be required to file an answer within 30 days after being served. She can only admit, deny or say she doesn't know. The first answer is easy...but the logical next step if she denies or says she doesn't know is a court-ordered DNA test. An experienced attorney can walk your son through the process on that issue probably without difficulty.