Why do you want him to buy the 1x9?

And what's that about clearance?
Do you spend much time log jumping?

Clearance is only ever important for very special kinds of riding.

3x9 has a wider range.
Admittedly, I don't use the granny much. Snow riding, mountain touring.
I could probably be quite content on a 2x9.
But 1x9, no.
I'd either lose speed on the easy stretches - or riding to the ride spots - or have to put up with fairly big steps between gears.
Wouldn't be particularly nice for my kind of riding.

Simpler - not much.
You lose the left shifter, that's about it.
Many 1x bikes need some sort of chain guide anyhow, which is comparable to the front derailer.

And are the bikes otherwise comparable?
Brakes, forks, wheels, fit, size?