Hello,
I've kept donkeys for 30 years. Jacks, uncastrated males, can be dangerous.
It would be a blessing to him for the rest of his life, maybe 40 years, if the community could raise the money to have him cut. I wouldn't hand feed him treats, but it might be worth putting down a dish of feed for him. I wonder if he can be tied up, if so you could combine feeding and tying him up while you are in the paddock. He should be getting his feet trimmed every 6-8 weeks, if this isn't happening he will quickly become an animal/neglect cruelty case. If he is getting his feet trimmed have a chat to the farrier to find out how he handles him. From what you say about the owners I suspect his feet aren't looked after.
You might like to investigate The Trust Technique with James French. You can learn it online, I did it several years ago, it works. You could use it from the other side of the fence to begin with and then in the paddock. It's a way of building trust and teaching boundaries. Good luck.