Well, I can tell you that CNR wouldn't like this response, but you know me better than she does.
WW, I think you need to let him make the decision for himself. If he runs and is successful, then you can congratulate him. If he runs and is not successful, you can sympathize with him. Don't set yourself up in this situation as to telling him he should or shouldn't. I think that as an individual, he has to make his choices and then suffer the consequence whether they be good or bad.
The ultimate outcome of this, is that you and he aren't in an argument and he isn't feeling like you're trying to mother him.
Personally, I agree with you, but I'm not running one moment let alone 9.3 miles.
Good luck on your run, and focus on your accomplishments. If both of you reach the finishline, then you are a family of success.