You’ve got to go on the offensive. In the United State, the truth is its own defense. Label him a sociopath. Demand he submit to a court ordered brain scan (plus a DNA sample).
Perhaps the most useful test would be for the incomplete brain wall, the cavum septum pellucidum (I’m thinking out loud here, not offering this as expert opinion). We need to find out which test is the best for courtrooms, one that would give a yes/no answer and that would be easily visible to an informed layman in side by side scans. Anybody know? Adrian Raine’s tests are mentioned in these posts, https://pathwhisperer.wordpress.com/2011/11/29/criminal-minds-will-testing-the-brain-even-before-birth-separate-the-good-seeds-from-the-bad/, https://pathwhisperer.wordpress.com/2011/09/20/post-redux-are-saps-socially-adept-psychopaths-real/. More, Abnormal Brain Region Characterizes Those With Psychopathy, Habitual Liar Brains Look Different On Scans, and www.futurepundit.com/archives/001998.html. Other tests also, https://pathwhisperer.wordpress.com/?s=brain+scan.