He's already filed a charge with the NLRB. And if his claim is that his memo was concerted, protected activity, which seems to be the case, then his exclusive remedy should be through the NLRB. (No jury.)

If he's successfully, his remedy will be limited to making him whole, which includes reinstatement (unless waived) and back pay and interest, plus possible job search expenses, etc.

In other words, he might win, but it won't be big.