Wow.. I'm not normally so long winded on these matters and I know someone is bound to disagree with me, but here goes...
So he's probably a few years old, you cannot get near him, he is basically wild. I would assume your FIL was feeding him wormer in his food. Does he not come to you for feed or do you just leave it out and he eats when no one is around?
I have to wonder why you took on this animal to begin with? I would have let the previous farmer put him down for his own good.
I don't mean to sound harsh but you are doing him no service by continuing to feed him and dragging this out for him. No one is going to willingly come along and chase down a wild donkey, maybe a meat man, but not anyone else and they certainly are not going to pay you for him unfortunately. His hooves are probably a WRECK if they have never been done too.
Did you guys never think of sedating him for hoof trims and teeth floating? Sneaking tranquizers in feed or having your vet shoot him with a tranq gun?
I would say at this point to get a vet involved, slip some sedative in his feed and then euthanize him while he's out.
These are not solitary animals, if he has no herd or friends he is living a mighty lonely life, coupled with no human contact because some A hole didn't socialize him as a foal (not your fault), and the possibility that he may have pain from overgrown hooves or hoof problems and you have a pretty miserable animal.
I don't think it's your fault he is the way he is, but at some point you need to ask yourself about quality of life here and when I'm assuming you rescued him from being shot by the prior farmer, you took him on as your responsibility. That means it is your duty to find some way hell or high water to make sure he gets the proper vet/farrier care he needs.
If you financial situation were better I would tell you to get him knocked out, vetted, floated, and his feet done, and then go buy him a VERY human friendly companion donk or pony to learn from...
I just can't see anyone willingly wanting to take on another mouth to feed that they will never be able to touch, pat, catch, etc. A pasture ornament of sorts, not in this economy when fully trained valuable animals are literally being given away.
I really feel for you and I'm sorry but someone needs to be realistic here. You say you have food for several months, then what?
Do you think it's fair to call a rescue into this situation when there are so many other friendly and well trained horses needing rescuing? I'm not normally one to say one life is more precious than another but this may be the time to think fairly. If your wild donk is taking up a space in someone's rescue that's a space another horse who may have a great promising temperment and be able to adopted right now, cannot fill. Your guy may fill that space for months waiting for the right person/training/etc. when several good adoptable animals could have been in and out in that time.
I just don't know. It's alot to think about.
I'd love to know more about his sitaution. Does he have shelter? Are there any other animals with him? Is he protective of them? Is this a full size donk or a mini? I really feel compelled to help you or support you in any way and I'm just trying to provide you with a level headed opinion from someone outside of the situation. I know many people in horse rescue but I just don't know if it would be fair to call on them for something like this. Feel free to PM me and I hope you have a safe and easy delivery and a healthy baby!