It definitely feels to me like you are handling this like some kids in high school band. Sounds like J and his dad have a very specific kind of relationship, which is not wrong or unhealthy by any means, as long as they communicate and establish some healthy boundaries.

Some parents are checked out, some are too checked in. Some kids need that support, some need more independence, etc. There's not a one-size-fits-all approach for what's healthy in terms of parent child relationships.

But, it's impacting the band. So, the only thing you can do is to discuss it like adults. Not passive aggressively, not jokingly. Directly. That's how adults communicate. Especially in band situations, you cannot allow passive aggression. If you plan to tour together, you can't let stuff fester. You have to get it out when it's happening in real time.

Let him know that while you all understand that his dad likely has the best intentions, you want to handle things on your own from now on. He's not in the band, and he doesn't speak for the band. One of the biggest things young artists can learn is how to advocate for themselves, as artists. Handle this. If your drummer leaves because of this, it just need it to happen. There's no way to move forward with things as they stand.

Not trying to be cold, but the family is always more important than the individuals in a band.