Don't think that I didn't think about it. Looks like we are about 2 hours..Maybe a day trip
It will not be easy. My first suggestion is to get a halter on him that fits well, but is big enough that if you have to leave it on for a few weeks that he has room in it. Attached a rope to it that is just a few feet long. Just long enough so you can get a hold of him, but not so long as he can get himself caught up in it. I used to braid up some baling twine and make it about 2 feet long. When you are putting it on him try to rub his neck and down his back so he gets used to being touched. It make take a little while to get him used to putting the halter on and off.
As long as the Momma is okay, you can try to lead him around the mare in a nice contained area. If you aren't comfortable in a small area you can do it as you walk them out, but it will be a 2 person job. Someone needs to lead the mare and the other needs to handle the foal. When you try to walk him for the first time he is going to try to pull you all over the place. Just hang on. Keep a steady pressure with the lead rope. You will probably need to walk with him as he pulls backward, but you really need to pay attention to his body language so that the minute that he shifts his body weight in the direction that you want him to go that you release the pressure on him. It is a positive reinforcement. It will not be a fast process, but in a few weeks he should be walking better. Just make sure to watch yourself and I recommend gloves so you don't get rope burn.
I think that we are only a few hours apart. If you need help, maybe I could come down on a weekend to help you out.