I think Calvin is asked on the journey with Meg and Charles because maybe, who knows they might need him or he’d be an important part of the story! He’s there because he feels he needs to be.
Calvin is asked to go on the journey maybe because he is important like I mean maybe they need his help or he became their friend so they asked. They seem to trust Calvin. Charles maybe knows that they will need him.
I think he went because he wasn’t feeling right at home with his family and when he met Meg and Charles Wallace he liked them and felt connected. He wanted to experience this journey with them and help them find their father.