The son is just a notch or two above scum to get himself in that position and should seriously pursue a vasectomy (At least he's holding down a job, even if it is a dead-end one, and paying some pittance toward the previous two kids). However the only way he can sign away his support obligation is by consenting to an adoption. His "Signing away his rights" means exactly Jack Shit with respect to his obligations, short of an adoption.