Basically whatever you teach is about breaking it down & starting with the easiest bits, then with success & lots of repetition, positively reinforcing(rewarding) the smallest tries, you can gradually ask for a little more, until eventually the horse reaches your goal.
Ask him to put his head down as low as he will. Give him a treat when he gets there. With lots of repetition, get him to put his head gradually further down & between his knees and hold it for a bit. This is often called a 'carrot stretch' because a valid way of getting the behaviour in the first place is to 'lure' it with a carrot<G>.(Don't continue to lure once he gets it tho). Get him doing that reliably on cue before picking up a foot & asking for the same. By that point, he will probably just put his knee on the ground when you ask him to bend that far, so reward that with an Extra Special Treat.