Make sure he realises legs mean the same thing as the crop. Start by squeezing and when he doesn't respond squeeze harder, If he doesn't respond to that tap him straight after with the crop to get him moving. Very soon if you do this constantly he won't need the crop so often to move forward as he will move off the squeeze of your leg. You may have to repeat the exercise every now and again but once he associates your legs with forward then you can introduce the other whip etc xx
He doesn't associate your legs with forward, just the crop. My horse is the same, it's a training hole. How I would progress is drop the crop and try Spurs. Touch, squeeze, kick, and then toe down and light spur, rinse and repeat.