Yep, always a tricky one, this.
But you and the rest of the band have to decide how serious you are about the band and making it work.
A previous post suggested your singer telling him, as she's a good friend of the drummer.
You could all get together, maybe over a beer or a cuppa, to tell him as a band. But be nice.
You don't want to seem like you're ganging up on him (which he may well think, either way).
Good luck, whatever you do.
Cheers
Steve