Give him a squeeze. No response? Give him a kick. No response? Give him a smack behind the saddle with the whip.
That's the best method of making them get up and go. It's worked for me countless times, and is much nicer for them than constantly kicking them.
Lazy/stubborn your just going to have to make him do what you want, I once ride this girls pony for her because she just wouldn't go forward, I got on, grabbed a stick from the ground, ask nicely, then just had to tell her, she was just plain stubborn, got her to move in the end was kicking her pretty hard mind you