Is he alone? I'd give him someone to keep him company even a goat or pig if you don't have a horse or cow that you could put with him. If your scared that he will slip outside in the winter maybe you can look at changing things around this spring/summer/fall that will give him a safer place to go next winter so he can be outside and not so board (just a few thoughts). I have tried the bars of soap and it works, I have also used metal and it works, they try to chew on it and notice it's not the wood and will stop before any damage to their teeth - even horses are that smart so you're donkey will know asap! But I really do think he is lonely, he wants some company or to go outside. Good luck and good on you to be so concerned about him too!