Have you tried riding him with a whip or crop? Kicking him hard can make him even more unreactive to your leg. A horse can feel a fly landing on its shoulder, so it can sure as hell feel a light leg aid from it's rider, kicking them hard in the ribs can also bruise, and even BREAK ribs. Kicking hard is not the way to go.
I would ride with a dressage whip, so you can tap his hind leg without having to change your position. Start with halt-walk transitions. Put your leg on lightly, and if he doesn't react give him a flick with the whip - not hard, just a little tap should startle him into moving forward. Maybe hold onto the saddle with long reins, because if he jumps forward you DO NOT want to hit him in the mouth accidentally with your reins!! Any forward movement is good, and you can refine it later once he starts moving off immediately from the light leg aid. When he will walk off from a light leg aid, do the same for walk - trot.
When he is sensitive to your leg in transitions, he will become more sensitive within the pace ;)
There are a couple of different ways, you could also try 'breathing' your leg, so putting your leg on lightly, and if you get no reaction, take the leg completely off his sides, move them back slightly, then put them back on. Often you'll get a reaction from that as you have taken the pressure completely off for a second.
You could also try using your reins on his shoulders. Put your reins in one hand and rest then on his wither. Then with your other hand, flick the reins back and forth across his shoulders until he takes a step. Often you can get them so sensitive using this method that they'll move off as soon as you move your reins!
The key is to keep the aids light, remembering that they can feel a fly on their skin, so there is no need to kick them hard to make them move.
Make sure you're not pulling back while you ask for him to go forward as well. Many people get frustrated with their horse for not going forward, but they've got the 'brake' on at the same time - I had that problem when I was a kiddy, I wanted my mare to go forward, but because she'd bolted on me before I was too scared to let go of the reins, poor mare!
Also make sure you're not gripping her sides with your legs constantly. Your legs should hang along her sides with the contact of two wet towels. They should have no grip on her, this will only deaden her to your aids. I ride with my legs hardly on my horse, and will only engage my leg when the horse starts to deviate its gait or direction from the one which I have set with my back.