How do I engage someone who is playing his character poorly?

I am DMing a game where one player has never quite seemed happy. He regularly questions rulings and gets quite hurt when he tries to do things with his character and they don’t succeed.

He is playing an elf rogue/warlock, but consistently tries to play the character like a tank, running to the front and attacking. Many of the encounters I design have a high (difficult or near deadly) CR but include puzzle elements to allow the party to win them either by achieving their goal without killing all of the opponents or by using the environment to their advantage (one was designed to give them an advantage in pushing opponents off of a high ledge, another included grease bombs that they could set on fire).

While the rest of the party tends to squeak through these encounters and have fun while they’re doing it, he has died five times. At each death I’ve come up with a way to revive or reincarnate his character, but he is still upset. I have given him numerous opportunities to roll a new character that fits his play style, but he has rejected them. I obviously cannot tell him how to play his character, but I’m at a loss for how to make the game feel fun for him, as well as the rest of the party that seems to enjoy it as is.

When I have tried to guide his character and the party more closely, he has complained about railroading. He does very little roleplaying and instead focuses almost exclusively on his stats and abilities. Put differently, how do I engage a player who is playing his character poorly?