If the individual parts are open to interpretation, I'd at least hear them out. Otherwise, it's "your parts are open to interpretation. MY interpretation." And if it's too much just be blunt and say so. He'll either find happy medium between what he wants and the band wants or he'll get frustrated and leave. I much prefer the approach your band takes to individual parts vs the single overlord approach, but it only works if musical tastes are very similar. Perhaps you can write a filler song for the album that lets him go Keith Moon to get it out of his system.