If you like working with this engineer and want him to do this phase of the work, listen to the other ACOUSTIC acts you mention he has recorded. It should be pretty simple: if you like the way one of them sounds, and the music is comparable to what you're doing, tell him that's what you're after. If you really don't like the final sound of any of them, that's a pretty big tipoff that while he did a great job on the recording, someone else would be better to mix & master (or, combine his expertise on the qualities of the recorded tracks with someone else's skills at the mixing board, this can work quite well if the someone else is both technically conversant and able to understand what you're getting at).
I wouldn't worry too much about wanting your own sound. If the sound you make is unique, it won't come out the other end sounding like Brittney, will it? But it's key to have someone who really understands natural acoustic sound, starting with an understanding of the kind of music you're making.