Rescue a house martin

I left my house this morning to see a young house martin on the floor. It doesn't appear to be hurt but It hasn't learned to fly yet. We tried putting it on top of our garage so it can fly away to strengthen its wings however it didn't get very far. I have put it into a box as my cat would eat it with some water and flies but it hasn't moved. I was wondering what the next best step is to take care of it until it has learned to fly.