How do you tell someone he is a bottleneck? How do you change his behavior?
Start by invoking common purpose. Often someone becomes a bottleneck when he loses the sight of the common goal and focuses on his functional role. We want to reverse that.
Second, tell the bottleneck he is very important, because he is. He has the largest amount of work. His pace controls the pace of the entire line. He knows the many important things about the process, because he is usually acutely aware of the problems around him. And a change in his behavior makes the largest change in the output.
Third, introduce the problems unnecessary bottlenecks have in general. Inventories surmount, feedback cycles get longer, information is lost, and other parts of the organization lose energy. Maybe draw a simple value stream map of the current situation.
Try to avoid the feeling that the bottleneck is personally a problem. He (probably) is not. Processes develop bottlenecks. Processes that are not actively maintained develop bad bottlenecks. Thre always is a bottleneck and this time it happens to be him.
Whatever the solution, it should come from common understanding of the situation. It is unwise to tell the person how to work. The change is in him.