He's still very young. As adults, they eat a lot more veggies, about 80% of their diet. As juvie's, it's only about 20-50%.
You can try mixing it up, but it's not cost effective for us, right now, to furnish our little girl with different veggies.. we just feed collard greens.. .99¢ a bunch here.
I wouldn't worry too much, if he is eating some. You may stumble upon something that he really likes, though.. some can be very picky eaters!