Garret wasn't a great drinker. He could handle his booze and even enjoyed it but never indulged in excessive consumption. Still, Garret couldn't help but go slightly overboard that day.
Khan was no liar. Sure, some of his political achievements and maneuvers were sketchy, and his skills had little to do with humankind. The same went for the data about his body, but his character remained stubbornly honest. He wouldn't have caused so many messes otherwise.
Moreover, Garret was a scientist at the peak of his field, and his family had gathered all kinds of medical information that would never hit the network. He knew about the Nak and some of the side effects of their mutations, so even the reasonable part of his mind couldn't dismiss Khan's revelations.
