How do you teach your horse to kneel A first impulse on what to do is somehow force the horse to kneel but forcing will not teach the horse anything other than be more afraid. So lets break it down. We know horses can kneel because we see them do it before they lay down. What can we do to make it so the horse wants to lay down? Getting them really tired may work but they probably will outlast us since they only sleep a few hours a day. So why else does a horse lay down? The answer is to roll. And why do they roll? Because their back itches! They key will be to make them feel itchy on their back.
Usually after we ride or give a bath and then provide a soft spot for them they will roll and before they roll they kneel down. So give your horse the opportunity to role after a bath or ride but have him attached to at least a 20+ foot long line with you on the other end and be patient while you wait for him to attempt to lay down. Have a treat ready too. When he kneels down go over to him but be very careful not to be in the line of fire of his feet in case he suddenly jumps up. Give him his treat. Now you may not get to him in time before he does the next thing after a kneel and that is a roll. That is okay but give him his treat anyway. This hopefully will stop him from rolling more as he is occupied with eating. As soon as he gets done eating wiggle your rope lightly so that it annoys him slightly till he gets up.
Repeat this routine after each ride or bath. Eventually you will be able to give him the treat soon after he kneels but before he rolls. Also only wiggle the rope when he rolls and not when he is kneeling. He will like that he keeps getting a treat when he goes down and so he will start to keel down before you do something that itches. He will also learn that the kneel is what is desired and won't attempt to roll. Once he is doing this start applying a cue in addition to giving out the treats.
All of this will take numerous sessions and so if you only cause something to itch his back once a week be prepared for him to take months to learn to kneel on command.