As others have mentioned, his spin rate plays a part in keeping the ball up longer than an average fastball. He also throws almost straight over the top, so one would think he imparts damn near 100% backspin on the ball, again helping it resist the force of gravity longer than average fastballs. Lastly, he gets down the mound well and with his long arms, he’s releasing the ball closer to home plate, reducing the hitter’s reaction time. All of that combined with his ability to place the fastball where he wants make his 90 mph fastball look like 95.
Since he can dominate with a fastball 80% of the time, hitters get geared up for it and that allows him to mix in a decent curveball and a plus changeup.