I've had success using Colorcard pictures with games (child imitates a target phrase) for more structured responses. I also use Fisher Price playsets and engage the child in play and 'bombard' with the target auxiliary/copula. I usually mix the structured with less structured activities. It also helps when the family reinforces what you are working on. I think that's why I've had such good results with the little guy I'm currently teaching the same skills. Good luck!