Can you post a picture so that we can see what age we are talking about here? Right now, keep it in a warm and quiet place...a hot water bottle wrapped in a towel or a heating pad on low under half the box or cage that you are keeping him in...so that he has the heat source but can get away from it if needed....the picture will help because that determines what and how you will be feeding this little baby!! Thank you so much for helping him!!
Is he pooping? What are his poops like? The condition of his poops will determine whether he needs rehydration before feeding.
As for food, if he is a fledgeling rather than a nestling you could try offering him small bird seed. Scattering the seed around him might encourage him to pick it up. He will also need water. If you place warm water in a bowl and dip his beak into it he will most probably drink.
If he doesn't feed himself you can defrost some frozen peas and corn in a cup of boling water, gently prise his beak open and pop them in one at a time. Or you could try slipping his beak between your fingers to see if he will associate this with parental feeding and gape for you, so you an place the peas in his mouth.
Can you tell us what country you are in and where he was when you found him?