Okay, first thanks for all the help and advice. Second, let me get this a little cleared up, sorry for the misunderstanding in the first place.
1. He's not tied to the tree for longer than half an hour each day, the rest he spends in the field. He has food and water at all times.
2. The field is now fully fenced.
3. The other horses also only spend the same amount of time in the field, all with food and water, before going into a stable or the field afterwards. As it is a riding school, it would be hard to keep recatching all the horses all the time.
So now on to the more information thing. He hasn't been taught a head down cue, so I'm gonna start working on that. Should I try to explain this situation again?? Either way, I bought a lick it and some polos to help but he was good this week, so it does just depend what mood he's in.