Help fixing Pop ups?

Could be many things. In addition to the suggestions from Noah and Old Ag, here are a few other possibilities:

Is he popping up high pitches that are out of the zone? Solution: swing at strikes (nothing above the heart).

Is he's popping up pitches that are high but in the zone? Check to see if he's dropping his hands as he triggers. Solution: drive the hands straight to the ball, with no dip.

Is he swinging a bit late? This can mean that he's not quite getting the barrel through to hit the ball out front on middle-in pitches. This causes lots of popups. If so, there may be a mechanical problem that causes his bat to lag (not clearing the hips, too much tension, too much extra motion in his swing, etc.)

Or early? This can force him to slow the bat, and that leads to popups, too.

For some players, it's merely a problem of optical illusion. Let's say your player has been facing pitchers who throw the ball about the same speed. His brain has locked in on the normal trajectory of the ball, which moves more or less downward. If he's suddenly facing a faster pitcher, the trajectory of the ball will be flatter (it may even seem to rise), and he simply hasn't trained his brain to adjust yet. If I sustect this, I tell the player to "hit the top of the ball" or "hit a hard grounder" until he makes the adjustment.