You have a very big heart, good that you did the right thing and rescued him, because of his size he should be a few weeks or days old, this moment is crucial for his development and it is important that you keep him well fed, try to be aware of when he wakes up so you can give him his bottle, if you keep him well fed he is more likely to survive, take good care of him and get him a comfortable box to rest, keep taking care of him, his life depends on you now. I look forward to learning more about him, blessings to you both!