Flamestrike
Legend
For the sake of argument, let's go with that - hiding and remaining hidden are two different things. The RAW we're interested in is "You can't hide from a creature that can see you clearly...".
If I revise that in the simplest way to draw the implication, it's "You can hide from a creature that can't see you clearly..." so I am curious about your arriving at ""You can remain hidden from a creature that can't see you clearly...". Why do you resist the former and embrace the latter?
He agrees you cant hide in dim light under direct observation (barring a special ability that allows you to do so).
But in the same breath he also asserts you can remain hidden in dim light, even while under direct observation.
I think he's trying to turn his house ruled 'perception check as a reaction to the creature entering dim light' into the 'direct observation' bit. Or in his mind, a successful perception check translates to 'you observe the now formerly hidden creature'.
Which is putting the cart before the horse. If you can directly observe the creature (which you can do in dim light or light rain or snow just fine) you dont need to make a perception check at all.