How do I retrain my first parakeet to sit on my finger after getting another parakeet?

I had a single parakeet for about 3 1/2 months. He was trained to sit on my finger and loved spending time with me. He would jump on his cage doors when he wanted to come out, which was quite often. I got another parakeet and now the first one won’t sit on my finger anymore. He constantly has to be next to the new parakeet and won’t let her out of his sight. I know this happens sometimes with two parakeets, but I miss having a trained parakeet. How can I retrain him?