I thought I would post a follow up message.
Things have been a little better over the past few days. My horse is spending less time crying for his buddy and a little more time being farther away from the fence separating them. In the barn, the little guy is staying in his stall with the door closed and there has been no fuss.
I've been working him close to the mini and spending time leading him around the field, farther each time. I bring him a treat every time I go to see him and have had no troubles putting the lead back on him.
He has also stopped chewing on his stall. I think this is great progress for such a short time.