Picky eating common. Picky eating is very common, most often meats or vegetables. Some of the other answers gave you some clever suggestions; i'd also use a multivitamin as some insurance. One last word of advice--continue to offer them, but don't force them nor play games to try to get them in. Neither works, and either way you'll wind up with a behavioral problem worse than the feeding concern you began with!
Keep trying. Green leafy veggies are a critical and important part of a healthy diet. Keep introducing different veggies (spinach, broccoli, green peas, etc). It takes many introductions to get a child to like a new food (or sometimes to even try it!). You can also hide veggies in sauces by pureeing them first and adding them to tomato sauce to put over pasta or chicken. With each meal offer a veggie.
Food fights. Every food taken in moderation is necessary for optimal growth and development! Toddlers know how to put up a good fight when it comes to feeding time. Do not come across aggressively. Continued presentation with carefully placed incentives will almost always get you your objectives. The sky will not fall if in this phase of development he refuses greens, vitamin supplements might help. Take care!
Keep trying. Another way to incorporate green veggies into your toddler's diet is to add them to smoothies. If you make a smoothie with fruit and veggies your toddler will not taste the veggies.