Well, if he can't eat it outside, I have another idea:
Put the grain in his bucket. Let him eat while you are there, but walk all around him, all around the stall -- go where you don't normally go - as far away as you can without leaving the stall. Do the same thing tomorrow, but open the stall door. Try to spend time standing at the stall door while he is eating. Basically, gradually teaching him that it's alright and safe for him to eat without you being RIGHT THERE. Then step outside the door and watch him. Then walk away and come back... get the idea?
Just make sure he is eating before you start wandering around and don't rush right back to him when/if he stops eating. Find his comfort zone for that day and move just outside of it.
Is he OK in his stall any other time? Sounds like its the stall he has a problem with, not the food.