A few basic tips can sometimes be helpful. Of course, not seeing him in action makes it hard to determine which one(s) would fit. Try some of these...
1. Improve footwork. Most error (even throwing errors) are caused by wrong footwork. Focusing more attention on the footwork to the ball, through the ball, and following the throw will take much of the focus off the throw. The throw is a byproduct of the footwork. Focus footwork and often the throw will take care it itself.
2. Follow the throw to 1st in a straight line. After throwing, he should be taking a few steps directly towards first. Some guys peal off left or right or even just stop after the throw. Keep moving.
3. Don't tap. Some guys when they think a little too much will not just catch and throw. They will catch, tap the ball in the glove a couple times, and then throw. The extra time it takes to do that can throw off the timing and allow the player to think about it even more. Focusing on the footwork can help with this too.
I'm sure others will add some good stuff as well.
Hope this helps!
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