Let me start this dialog and let others jump in. I would try to spot his specific cause of inconsistent delivery and remind him to work on that - could be foot or leg, glove arm or throwing arm or shoulders or head. But knowing what it is externally and communicating so he gets it...?

After looking at where the location of his pitches are going, I ask my catcher to verify just what is the umpire calling. Are his pitches close, are they strikes that are not being called, is his strike zone really tight, etc. After establishing that the problem is not with the ump, I try to get my pitcher to relax, deep breaths, tell a short joke. Any thing to get him to relax. Then I try to pick out one thing for him to focus on, such as arm slot, staying tall, stride front foot towards catcher, etc. My personal philosophy is that you teach in practice and encourage in games. I try to keep it simple and hope that he can make the proper adjustments that will enable him to stay in the game. I coach a 12u travel team. Jim Broughton

Both great answers, getting the pitcher to relax and think about his fundamentals and indentifying any obvious problems is a great start. I always tell them there's no clock in a baseball game, slow the game until you feel comfortable, reintroduce yourself to the infielders, give a smile ( a lot of pitchers start to get tunnel vision when things are going bad) and as Jim says "relax and let's have some fun".

He must relax and think about what he knows to do. We have practiced all he knows to do. So in a game cue words from the dugout allow him to focus on certain adjustments I see he needs to make.