i'd start with using baby carrots to encourage your horse to lower his head to the ground. if you want him to move his foot out and really bow, just stretch the carrot a little further. only give it to him if he bows, not if he just walks forward to take it, and if you want to use a word-command, say "bow" only when he does the action you're looking for. remember, it's going to take a lot of practice!