Talk to the entire group, like "hey guys, let's watch our lines and pull through smoothly" That way you aren't calling one guy out. It is a safety concern, and if let go you'll have guys who simply won't follow a guy like that in a paceline and you don't have full participation. Everyone is in charge of their front wheel.
This is easily fixed with relaxing arm tension. You can't just tell him to "hold a straight line" That is like a little league coach yelling for his pitcher to "THROW STRIKES"
You've got to give him the tools to change. Slack your arms, change your gears while pulling through. Look back through your torso and arm instead of over your shoulder to see if you are clear.
This is important because safety is important.