Found a baby Great Kiskadee...

[Pawbla]

Well, right now I have him in my old cage that I used to have for my bugdies. I'll move him as soon as my father removes the stuff that is in the cage that I use for rescue birds. His body is almost fully feathered but it's still too small and his wings haven't got all their feathers.
Well, I rescued this little guy from a park. Yesterday I went to the park with my boyfriend and saw two little babies and their parents. There were also four chimangos (some birds from the falcon family, if you don't know them), so I said that I'd leave them there and if the next day I found just one, I'd take it.
So, today I found this little guy very cold, next to a dead dove. No signs of the brother. I got him home and I am providing him heat. He seems fine now, he has just made their typical sound. I am afraid he'll gt pretty nervous in any moment, though xD.
I know these little guys are omnivore, so what should I give him? Preferably something that doesn't have to be bought or my mom will kill me xD. I have some soy drink that is pretty nutritive, and doesn't contain lactose. But from what I've heard it's a bit acid (like coke). Would that mixed with something work? And what could I mix it with? If not, what could I feed him.

Also, I'm going away on a trip from Friday to Monday. What should I do?