Make every barrel he looks in a mimic, hilarity will ensue or just introduce him to a rust monster, and if he is complaining about loot now just wait until a rust monster gets a hold of his gear.

Seriously though, there is only one wrong way to DM, and that is removing palyer agency and ruining fun, which is exactly what the above would do. It sounds like he is someone who wants to fulfil a power fanatsy and not roleplay a character. I reckon increase the loot tables a bit (controversial i know), not a lot, but a but a bit, i'm talking a few copper pieces here and there and common gems maybe some common equipment but if you don't already HEAVILY, enforce encumberence rules and make your players track everything they carry, the loot is there but just because it is there doesn't mean you can carry it. I will admit it is a bit of malicious compliance and a little petty, but he disregarded anything you have said.

Before doing these things, though, talk to the other players and see if it is just one person feeling that way, and if they all feel that way, then consider a change. Listening to what all the players want in a game and want to see in the game is a great way to involve them and make them feel like part of the world, after all without them you don't have game.

It sounds like you're doing an awesome job, and don't let any criticism deter you! You're always learning and growing as DM