I'm very relaxed and enjoying life. But I also know that pitching injuries are serious business and so is developing healthy pitchers. There's no magical formula and even if you do everything by the book (whatever that book is) it can still go wrong. Which begs the question - why take unnecessary chances?
I'm not trying to be difficult or a jerk but re-read what you put. You justified your 15 year old son pitching 130 pitches just to win a game. You're going to develop a sign to have your son tell the coach he's too tired. Which if he's not too tired then it's a lie. The more productive thing is to have your son have a conversation with the coach. If that doesn't work then you have the conversation. If that doesn't work then go to the AD. If that doesn't work then then you need to know everything you need to know about all the people at that school. At this point you may need to remove your son from the team. If it gets this far you've stuck by your convictions and taught your son about character instead of lying.
Just saying
TPM Happy New Year to you and your family as well. I would love to post more but I'm staying super busy doing the AD job.
I don't think you are relaxed. You sound very uptight.
And if you are not trying to be a jerk you must be a natural. 
I added humor to a situation that I found alarming. Thus the "lol". That means "laugh out loud". You can sue me for getting caught up in the moment of a game.
Concerning the sign, absolutely would I implement this sign IF after talking to the coach he ignored my sons' health. It's not like he is stealing crab legs from supermarkets or raping girls on campus. 
If that doesn't work then I would talk with the Athletic Director who is the head football coach and tell him that if he wants his starting QB to return next season then he (AD) needs to talk with the baseball coach. Football is King in Texas! 
Concerning his arm health I believe I should be the one approaching the coach.