Rib Strain

Did a google search on rib injuries a couple days ago.
Came up with “Rib Strain”.
Right outta the shoot, the first practice for our team, they went thru a few minutes of stretching excersizes. Then started throwing drills.
With the knee on the ground and just using upper body throwing, my sonn started feeling pain in his ribs. He’s not one to quit easily, so he kept throwing till he just couldn’t handle the pain anylonger.
There are as I understand it, three levels of “Rib Strain”.
Hopefully, my son has just level one, could be a 2 to 3 week healing process.
The coach is very concerned, as the boy has been the “go to” guy.
Next week, the team is starting a “workout” program for the next 8 weeks to get the boys in shape for the season. From what I understand, this paid workout instructor is going to work their butts off to get them in shape.
So, my question: What part of these workouts can my son take part in that doesn’t put strain on his rib muscles/tendons etc.?
He’s not going to like it if he has to sit out the first 2 weeks of training.
My next question: What excersizes should he have done prior to throwing the ball that could have kept this from happening in the first place?
Thanks in advance
ETA: These boys are 7th grade

If they truly started their practice by stretching, then they were asking for trouble. Stretching should be done when the muscles are already warm (like at the end of practice) - not to warm up the muscles. A good “dynamic warm-up” (google that term) will provide a proper warm-up. I’ve posted a dynamic warm-up routine on this site once or twice in the past so search this site as well. The gist of it is simply doing activities that involve movement and muscle recruitment.