Cards on the table for everyone. Just because you’re not gigging don’t assume that the project has no merit; the benefits to mental health of leaving the house once a week to do something that you love shouldn’t be underestimated. It may be that you all decide to wait for the drummer, and enjoy a weekly meet up and bit of a social, or you get a different drummer in.
No good can come from keeping secrets from only some of the band, and I think it’s unfortunate that the drummist has put you in this position.