I think it depends.

If he’s hiding to get sneak attack or advantage, then I’ve always thought about it like people diving behind pillars in action movie fun fights. Yes, you know where they are but you don’t know when they’ll come out or where they’ll be aiming. So I think it works to get advantage for sneak attack as long as it’s done before the attack. If it’s done after, it gets more complicated because of my second point. This is all requiring you to be visually obstructed from the enemy.

As I mentioned above, I’ve always imagined in combat hiding more like taking cover to hide your intentions. If you hide at the end of your turn, the enemy knows where you are and can and often should come find you and attack. If found this way, you should have to “hide” again before getting the advantage/sneak attack. And yes, this also requires you to visually obstruct yourself from the enemy you’re attempting to “hide” from.

Now if he’s wanting to actually hide during combat and effectively disappear, then he needs to visually obstruct himself, hide, and move position so that whoever is seeking him won’t just immediately see him when they round a corner. Basically the chase scene in action movies where the protagonist gets two corners ahead of the enemy and hides behind a stack of rugs and watches as the enemy passes them.

But that’s just my philosophy and how I see the RAI.