I know its A LOT of work, but you should be able to go to the court house and find all kinds of public records. If you can find the Uncle's son's name, you can go to the tax office and they HAVE to tell you of any land that is listed in his name, where it is at, etc. I would do a search like this for everyone involved. They could have Max hidden anywhere, but don't give up. You CAN find him. I agree with the others, you should try to get as much publicity on this as possible! If you don't have the time because of work or whatever, I would be more than happy to help, just PM me if you need anything. Mass emails, letters, phone calls, or whatever. Something needs to be done to light a fire under these people so you get your horse back. I just know that if my horse were stolen, I WOULD get her back, no matter what it took.
Good Luck and Hang in There!
Please PM me with names, counties, anything... I'll do some looking around at least. It can't hurt, lol.