If he is as I suspect an honest-to-goodness narcissist (as I understand them) - and I've had one such working for me (and it was no fun) - such people refuse to believe they are ever wrong about anything. If it sounds good and reasonable to him at the moment, then he will believe it...even if he believed something totally different in past months or years. Anyone who tells him he's wrong automatically becomes an enemy, someone to be crushed or at least knocked out of the way (got a story about that, too)...but at the same time, he honestly won't understand why the other person is so angry with him - he will have no sense of empathy. It really is all about him, and the very idea that he might be wrong about something or that the other person might be right - he would see the very idea as ludicrous.
And the real frustration is that he can lie his ass off, but he'd pass any lie-detector test...because he honestly believes he's telling the truth. It sounds good and right and reasonable to him at the time, so it must be (to his mind) factual and truthful. Even if you show him a clear video of him saying something 180-out, he'll explain it away...and even if his explanation makes zero sense to any 'normal' person, it would be eminently sensible to him - and again, he'd easily pass the lie-detector test.
A good psychologist could make a career studying Trump...and I suspect that some will as the years go by - I expect the movie within two, no more than three years.