Have you tried with a clicker? You can get them at kmart for about $7 and they are great! Most people use them for dog training, but they can work for pretty much any animal if used properly! It basically 'marks' the correct behaviour that you want from Chinga. Basically, when you pick up his front leg, wait for him to move the tiniest little bit towards the ground (even if just his nose moves!) then click the clicker and reward with a piece of carrot or apple. Make sure you click before you give the reward. After awhile, Chinga will realise that when you click the clicker he gets a reward. Every little inch closer that he gets to the ground, click and reward :) But remember, you must click the clicker at the EXACT moment he is doing the action that you want. It's important that you do it in the split millisecond that he does the action, or he wont associate what he has just done as being 'the good thing'.
I've personally never done this with horses, but its the same principles I use with my dogs and birds. It's a slow process, but once your horse associated the clicker with his reward, you can start doing almost any trick you like.