You don't really have to kick or tap the horse to make him trot. If he is responsive to voice commands you can ask him with your voice or encouragement. Also daily stretching and exercise will strengthen your legs and how supple they are. A good technique is, instead of kicking or tapping the horse, increase your leg pressure, wrap and squeeze your legs, with more and more encouragement, and don't be afraid to continually increase the pressure until he trots. Overtime this benefits because it uses and increases your muscles, and makes your horse respond quickly!