I know how weird that sounds, but cleaning a horse's pen can be a real bonding and teaching time, especially for a horse lacking confidence in humans. Whenever I have a mustang in training, I make a point of cleaning its pen every day, even if I do nothing else as a lesson for the day.
Think about it, if you put a horse into a pen formerly inhabited by another horse, the first thing they will do is make a visit around the pen sniffing all the piles. So, when I enter a pen and methodically go around taking an interest in all the poo lying around, this seems like perfectly logical behaviour to a horse and they immediately begin relating to me more like they would to another horse. It also gives me an opportunity to alter my human scent and smell more acceptable from a horse's point of view. If the horse is reluctant to get near me I will clean another horse's pen first and bring a partially filled wheel barrow with me to clean the mustang's pen. The desire to get a whiff of another horse's droppings is usually overwhelming and will induce a horse to come to stand near the wheel barrow. As I make trips back and forth from piles to wheel barrow it is the same as using approach and retreat. The horse grows more confident as it notices I walk towards it not with the intention of confronting it, but instead I'm just making a visit to the wheel barrow. Which the horse can totally relate to, I mean, a pile of dung in a transportation device is probably about the most interesting thing a mustang has ever seen in human land. In time the horse's curiosity will grow and most mustangs will eventually sneak up when I am focused on lifting a pile and they will begin sniffing the fork. Holding totally still and continuing to focus my gaze on the poo will usually give the horse courage to travel its sniffing up the fork to have a good of smell me. Which by that time I often smell enough like manure myself that I do not offend the equine nostrils. No matter how many times I make contact with a horse during training, nothing makes a more profound psychological impact than when the horse voluntarily approaches to explore me out of curiosity without any outside pressures.
When a horse feels like we are always asking for, or wanting something from them, it can be a real turn off, and actually damage the relationship we are trying to build. Simply being with a horse, not teaching anything at all can be one of the most powerful lessons your horse can learn from. Consider spending more quality time with your horse, and his poo.