Have you tried noise therapy? Every time he goes to munch on the plant, shake a jar or a can with pennies or small rocks in it. They hate the noise and will run away. Eventually, they should figure out that going near the plant means that horrible noise.
Or keep a spray bottle on hand and give him a little spritz when he tries to get to the plant.
The chain stores like Petco and Petsmart have cat grass that you can grow (at least they do around here). Maybe you can get him some of that to chomp on instead.
If nothing else works, you can spray the plant with Hot Pepper Wax. It won't hurt him but once he gets a mouth full of hot pepper, he won't want another one :)
Animal testing is a terrible idea; they get all nervous and give the wrong answers.