Try going into the field without the goal of catching him. He knows that's your goal. Instead, try making your goal something smaller. Like, say trying to pet him in the field. One idea I have is.. Horses do not need us to survive, they need food and water. So if you have hay in the pasture, go stand in/next to the hay so he has to come to you to eat it. It may take an hour before he goes to it, it may take two, but the most important thing to do is don't give up and walk away until he goes to it or you. When he comes near, act as if he's not there. It should spark curiosity and he may come to sniff you. When he does still act as if he isn't there. When he doesn't move away after doing so, reach your hand towards him and stop before he goes to move off. Also, reach your hand towards his nose, palm down. It simulates the way horses greet eachother. If he touches your hand when sniffing it, it's his way of saying you have permission to touch him. Then you can start moving towards him.
And personally once I'm able to pet him without him going away, I'd pet him for a bit, then leave the field for a while. It will completely blow his mind, and he'll think about what happened. Also, you may need to do this more than once.
Also, if you don't have hay in the field, water works just as well (although may take longer depending on his thirst)
I hope this helps :)