Spring mix, collard greens, mustard greens, turnip greens, dandelions, geesh the list goes on and on. Most any greens will work, even a little romaine won't be harmful. Just remember to do a variety. Does not mean each meal needs to be a variety, but as you use up one type of greens, try a different one next. Also work on finding weeds and such to feed, as they are so much better for him the store bought items.
almost forgot, cactus are another great option and can often be found in the store.