You’ll obviously have a fan ID as a previous season ticket holder but the only way to upgrade it to the same level as his as a cub, is to get a membership for yourself - or use someone’s that isn’t going.
To be honest - it’s a pain even as a season ticket holder, because on the day that cubs & members become eligible, at 5pm, you aren’t eligible to get a seat next to your cub! You have to wait til you can telephone & relocate your season ticket at 9am the following day, by which time, the best seats for kids have gone!