IF the coach had explicitly agreed to the terms of the email (i.e. that the son play every minute of defense) you would have a stronger case. However, the article doesn't indicate that this agreement occurred. It sounds like the coaches received the email, then called the father. I'm thinking that the coaches said to the father, "we recognize that johnny is great on defense, but we'd like to be able to use our judgment as to what's best for both johnny and the team." Quite possibly the coaches thought they had flexibility, the father thought they still would play johnny on defense all the time.
If this confusion occurred, do you still think the father was right?
And very possibly the commissioner responded, "You can play him wherever you wish, as long as he plays defense every minute of every game."
My understanding is that this is then what the coaches did. He played defense (and offense when needed). The final game, however, he played offense instead of defense. That wasn't what they agreed to. They were then fired.
"However, the article doesn't indicate that this agreement occurred"
Well, no, it doesn't. But why did the coaches abide by the written e-mail all season, only violating it in the final game? Coincidence?