If he only ate half a leaf, I wouldn't worry whether it's poisonous or not. On two sites, I only found that the fruit is poisonous. If he hate half a tree, he'll probably feel ill whether it's poisonous or not as well. Not sure what your post means, I hope he didn't eat half a tree!
Tony ate a few small leaves off a tomato plant (which is poisonous) and was fine, although we totally freaked out.