they say what do you get when you sober a horse thief?
a sober horse thief.
in your case, the lying wasn't directly caused by the alcohol he consumed, it's a by-product and what HE DOES.
you can't MAKE him tell you the truth.
you CAN accept that he is a liar. and that checking up on him, asking him, testing him, won't change a darn thing.
he'll STILL be a liar.