Don't get in the habit of "tapping" the horse to make it go forward constantly. The horse just learns to ignore your leg, because you're nagging him. Do your parents ever nag you? You just tune them out, right? Well he'll do the same thing. One strong squeeze should get him moving, and he needs to learn to keep forward impulsion. Carry a dressage whip, and when he stops and starts going backwards, smack him in the butt with the whip! He should move forward after this, and then you release the pressure. Your legs should be like a wet towel wrapped around him, they're there, but they aren't moving unless you're specifically asking him to do something. "Tapping" turns in to fidgeting, and then you become a less effective rider and it is a hard habit to break! :)
The hills were bathed in moonlight, the shadows not so stark;
Silver light reflected off his brown hide as he held me in the dark
– I love you, Mudpie!