Well I think this is the coaches fault. What was the priority of the tournament - win or showcase? If it was to showcase then he needs to stop worrying about winning and develop a plan to show the players so the colleges can see them. If it's to win then he shouldn't be using your son on the bases when he needs to be in the bullpen. Also, why did he put your son in without ANY bullpen time? I'm not the best coach in the world but I wouldn't have done any of this.
If the college coach is any good he will know that your son didn't get a chance to warm up. He's not going to hold that against him because it wasn't his fault he didn't perform well. That being said how was your son's velocity? That's what the college coach is worried about.
Have your son call / email the coach and ask if he can be seen again since it wasn't his best outing. Don't make excuses or anything just have him say it wasn't a good outing and would like to try something again.