No way they'll win this.
And the guy is selling more t-shirts than ever as a result of their stupidity and greed. Don't attack small guys with stuff like this since all you'll ever accomplish is wasting your time and giving them way more publicity and business if they choose to fight back. And if you win? You win nothing. In fact you'll have wasted time and money on them even if they back down instantly, probably more than you'd ever made by "protecting your trademark".
By the way I have no idea how someone selling "Eat More XXXX" t-shirts can damage them in any way shape or form. That's like saying that every time someone makes a "Where's the Beef?" joke, it steals money from Wendy's. People are still talking about the add 25 years later as a result of parodying the brand. The best thing that could ever happen to Chik-Fil-A is a ton of people makig "Eat More XXX" shirts.
But maybe that was their plan all along! Start a fight so it becomes publicized and people start fighting back by making tons of shirts! Yes! Brilliant!