I don't think there is any legal standing for him to ask for support and that you may be able to get the case dismissed on that basis. But in either case, you can argue that the son has voluntarily left your house. He is emancipated. Because of that, there should be no support order in any case. A "child" cannot simultaneously have the authority to leave the house (a decision for an adult, otherwise he is just a runaway), and also claim the need to be supported financially.
See the code here: http://www.pacode.com/secure/data/231/chapter1910/s1910.19.html
it reads, in relevant part with regard to emancipation
: (e) Within six months prior to the date a child who is the subject of a child support order reaches eighteen (18) years of age, the domestic relations section shall issue an emancipation inquiry and notice to the obligee
, with a copy to the obligor
, seeking the following information:
(1) confirmation of the child’s date of birth, date of graduation or withdrawal from high school;
(2) whether the child has left the obligee’s household and, if so, the date of departure;