It's classic Orwellian double-think.
JWs say they abstain from blood, but they take from blood (in fractionated form);
JWs say they don't follow human leaders, but they follow the Governing Body's lead (w.10, 9/15, pp. 21, 23);
JWs say they are not creationists, but they believe the universe was created.
Your dad can't back down even on this no-brainer, trivial matter. If he admits he's wrong on this, he'll also have to admit the Org's wrong on this and he senses where that could lead. You've more than adequately made your point. Having been confronted with the glaringly obvious, once your JW insists that black is white, you've gone as far as you can for the present. It's time to smile knowingly and change the subject.