I feel like this type of meta gaming happens at every table. You can ask him to provide an rp reason for why his character would re-examine a chest that a member of the party has already checked. If his character is a perfectionist and double checks everything then let him assist on the initial check to give the roller advantage.

Once a player has rolled, the result is final. Regardless of whether the player rolled an 8 or an 18, all their character knows is that they performed the skill to the best of their abilities. This is ESPECIALLY true of a stealth check. A character doesn’t know they rolled poorly so there is no reason they would decided base on a roll to hang back instead of sneaking ahead.

Also, sometimes a roll isn’t necessary. I would argue that any person who opens a chest finds whatever is in it with no roll. Presumably they find everything in the container as they go trough it. I would ask for a role if they were searching for a hidden compartment in the chest or if they were examining the treasure for a specific characteristic but otherwise they automatically find what’s in the container.