I wouldn't worry too much about his noshing the plants - unless he's showing any symptoms. One of the vets who post might comment but my understanding of plant-eating is that most cats simply wouldn't eat enough of the offending material to fall ill: the odds are that he 'mangled' the leaves rather than ingested them because they shouldn't be too pleasant.
(Lilies are one exception of course. (There may be others.) They're so particularly nasty because all parts are toxic so eg if they get pollen on their coats and groom off...........)