He will need food three or four times per day and it should be warm enough that you can just stick your finger in it.....be careful about only feeding him warm food because some birds will refuse to eat if their food is too cold and if the food is too hot to put your finger in, it is too hot for the bird to eat.
The watery poop is because you are feeding him cereal mix...birds will poop about every 15 minutes, so don't worry about that. When feeding him, watch his crop, the pouch on his neck, should fill up and appear fat...the bird usually will only take enough to satisfy its hunger, but your bird may try to eat a lot because he was hungry when you found him, and he may try to overeat.....feel his crop and if it gets too hard, hold back on his food until the crop empties out some.....
I assume you are feeding him from a spoon, so you need to be careful not to mix the food too dry (not enough water), because that can cause a condition called impacted crop, where the food is too dry for his system to move it through his body...a good mix might be where it just slides off of the spoon, but if the mixed food wants to stay on the spoon it's too dry and if it is runny like his poop, it is too wet.....
If you look around in the tree(s) under where you found him, I think you might find his nest...mynah birds like to push young young ring necked parrots out of their nest when the parents are away. I have a friend in Bahrain who rescued a rose-ringed parakeet that was somewhat older than yours, but he took it in and the bird's parents stayed around and the mother would fly down every day to feed her chick, even though the young bird was living in the house with my friend and his family.....the parents would come very close.....maybe as close as 2 meters, so take good care of your little friend.....
I will post some pictures of Filbert, the little rose-ring parakeet that got pushed out of his nest so you can see that a bird pushed out of its nest can be saved.....
Your uncle sounds like a person that can help you save this young bird.....
Good luck.....