I don't know much about them yet, as I just got my first one about 4 mos ago. I also have two mini horses. He was really skinny when he got here, so I started giving him 1/4 cup of 12% sweet feed 2x a day....he also eats grass hay with the horses. That must have been too much because now he has a big belly. I've cut him back to 1/8 cup each feeding. We'll see if that makes a difference. I have already found that they put on weight a lot easier than the horses do. Both of the horses are very easy keepers and get 1/2 cup 2x a day. I'm sure somebody who has more experience with them will be able to help you out.
OUr jenny here, who is a comeing yearling gets free choice hay, and at least 2 cups of grain a feeding. She is with the horses her age. That is alot of grain for a mature donkey as they do get fat easy. But for a baby I dont think it is.
This is my donkey I bought a year ago. He is the father of the mini mule in my avatar. I feed him 2 cps of Equine adult am and pm gets pasture all day and hay at night. He is of very good weight now, was a little thin when I bought him a yr ago. He loves to eat so I monitor his grain intake. Most donkeys do well on just hay and pasture but I just feel I need to grain him like my horses. Just be careful not to over grain, donkeys are prone to getting over weight. Also keep him on a good deworming program and have his teeth checked for points.