Need to teach him manners on the ground!?

Sometimes smacking them in the face when they bite can lead to a negative end and make them want to bite you more because they don't want to get hit. This horse has lost respect for you and you need to gain it back again. All I can say is do tons of ground work. For about ten minutes everyday, have him pivot, back up, turn, figure 8s, stop, and other random stuff to keep him on his toes. Ground work is a great way to form respect and a bond. Reward him when he doesn't good but keep him moving so he isn't able to do bad. Keep a close eye on him and whenever it seems like he's going to nip when your working with him or what not, circle him around before he gets to do so and if he is about to, just push him away so that he'll learn that there is no point cause you don't care what kind of crap he's going to pull. When you go to visit him, do you only ride him? He probably sees you only as an object that makes him work, not something he wasn't to please. Keep the activities you do with him random so he'll look forward to his visits. Take him for walks (yes like a dog) or just sit down and talk to him while he's chilling around so he gets used to your company when he isn't just working.