Maybee I'm just the odd one out but I love loud crashes. Our drummer can't play 16" crashes because they dont project enouph nor do they cut through the mix.
You also need to take into concideration what the rest of the band is playing. What I mean is...if the rest of the band is playing a couple halfstacks and a bass stack then a 16", though experience has shown me they sound better than 18", wont cut through the mix very well, or at least wont really stand out. On the flipside, if your bass player is playing a 100W combo and the guitarists are playing 30-50w combos...a 16" should be more than plenty.
On top of all that I guess it really comes down to what you AS A BAND want. Cymbals that stand out, or cymbals that simply cut through the mix to add extra texture.
He can try hitting a bit softer, maybee hitting closer to the bell (will add to projection and brightness but wont really "overpower") or possibly investing in some smaller crashes. But like state previously at the end of the day he is the drummer and you are the guitarist...the decision should be based on the bands opinion and not one persons.