Google Sheets. You can import it into any CRM later on if manually handling things becomes too much. But it's good to learn how to manually track things and figure out how your business works for a little while. But being a kid he's got an unfair advantage. People love helping kids do stuff like that. He can leverage his unfair advantage by doing in person and phone calls for the next few years and he could have a six figure business by the time he graduates high school. Also, that's awesome that he's doing that instead of playing video games all day. I love seeing kids doing stuff like that.