How to get a nervous dog to want to walk?

I am a new dog walker and my first client's dog is very timid and nervous when I went to walk him. He was perfectly friendly when I first met him with his mom, we even went on a little walk around the block. When I came to pick him up while his owner was not home he was very nervous and wouldn't let me put his harness on. I stayed for about 45 minutes to get him to trust me. He eventually took treats from me and let me rub his belly but ran away again when I went to get the leash. Any advice on how to get a nervous dog to trust you?