Commend him for wanting to be extra-safe. :thumbsup:
Sounds crazy, but he'll be more likely to listen if he knows you understand his goal and his priority.
Then point out that we KNOW, by hundreds of crash-tests and real-life crashes, how carseats perform with just LATCH or just the seat belt. We KNOW how the seat will work that way.
Admit that it's entirely possible that using both could be safer - BUT the trouble is WE DON'T KNOW. Heck, it's possible it would be safer installed upside-down, too. By using both he is gambling - it could turn out to be better, or it could just as likely turn out to be much, much worse.
Is he willing to take that risk, when there is a tried-and-tested method available instead?
And if he is still willing to take that risk, at the very least try to get him to get a good install with ONE method, and then just lightly buckle the other.