I agree with tinyliny - start with getting him used to moving off of pressure promptly and with energy. Pressure can be anything from a glare and more assertive body language to a physical touch from a hand, halter, or whip.
I recommend starting with establishing a solid go and whoa response in hand - take a look at some grooming and showmanship videos to see what the end result should be. Carry a dressage whip in your outside hand (do this on both sides, for balance's sake), hold the lead about 4-6 inches from the snap with your inside hand, and carry the coiled end in your whip hand. Look forward, not at the horse. Start walking forward, and give whatever verbal cue you like, whether a cluck or a word (I like "walk" - short and simple). If the horse doesn't move with you, keeping his throatlatch about even with your shoulder, get his attention with the whip. Don't nag him with it, or the whip becomes meaningless - the idea is to tell him to pay attention, something is happening and he needs to move. Make the difference between neutral-whip and move-whip very obvious to him.
To stop, exhale and allow your body language to soften (remember to keep facing forward - don't look at your horse, no matter how handsome he is
). Let yourself melt into the stop, and if your horse doesn't take the hint, pick up on the lead and block him with the halter.
The most important thing to remember is to do with your feet what you want him to do with his. A lot of people confuse their horses by asking the horse to stop while they continue to walk, or vice versa.
There are a lot of other groundwork exercises out there, but the basis of them all is a solid understanding of how to go forward and stop promptly. Eventually, he will begin to key off of your body language, and you'll need to remind him with the whip or the lead less and less. I doubt that he is truly lazy - he just doesn't understand pressure yet.