Chop up a small amount of grass into very tiny pieces. It might be easier to cut it with the scissors. Mix it in with the greens. If you moisten it so they stick it will be eaten better. Over a long period of time, gradually increase the grass and decrease the greens.
What kind of leopard and how old? The babcocki's don't seem to take to grass at a young age. You should keep trying if you can. Mix the grass, after finely chopping it, in with finely chopped greens. Spritz with water. He will have to eat the grass to eat the greens. As he eats this good, then slowly decrease the greens and add more grass. Keep doing this slowly, until he is eating only the grass. Good luck, let us know how it goes.