Once you have ruled out pain like smrobs said...
Try using a whip to back up your leg. Preferably a dressage whip since a crop is short and can't reach the hind without you having to stretch back and compromise your position.
Desensitize the horse to the whip before climbing on. Sack the horse with the whip.. lunge with the whip asking for more forward motion.. until he understands what it is.
On him, give the cue softly, then a little more firm, (no kicking) after no response, tickle with the whip. If he responds, praise and keep going. If he slows down again, ask with a soft leg cue.. no response.. whip tickle.
Now if he blows off your whip, then use it more directly. If he doesn't feel the tickles or blows them off, turn around there and whack him on his hind end until he moves. Make sure you don't say a word and you keep quiet and with him. You don't know how he's going to react to it.
But after you use the whip, and he responds... and then starts to slow, use your leg and give the cue softly. Don't just rely on the whip. The whip should back up your leg.
Also if YOU haven't used a whip before, practice holding it with your reins and don't get on until you can handle it without accidentally swatting him.
Now... if pain is not present... you need to do more groundwork. Get him to move.. with more energy. If you need help or not sure what to do with that, seek a trainer and ask on the forum :)
"Strength is the ability to use a muscle without tension"
Last edited by Skyseternalangel; 01-17-2012 at 05:35 PM.