One solution would be to revise the rider and gear load distribution. Consider having the guy carry some extra group gear or drybags to lighten the load on other boats that have to take riders he would have taken but is refusing.
Also, I agree with MountainMedic and CBRob. At least he's being up front with you instead of making this a surprise headache at Lee's Ferry.
I like to have a choice in who/what rides on my boat.
Nothing in the world is more yielding and gentle than water. Yet it has no equal for conquering the resistant and tough. The flexible can overcome the unbending; the soft can overcome the hard. - Lao Tse