Guy I’m seeing keeps accusing me of cheating and even tried to “test” me

I’ve been seeing this guy for a little while now, and almost from the start he’s been insecure about me supposedly cheating. I’ve never given him a reason to think that, but he brings it up constantly. It’s exhausting because no matter how much I reassure him, it never feels like enough.

Recently he decided to “test” me without realizing I caught on. He had a friend message me pretending to be flirty, just to see how I would react. I recognized what was happening right away and shut it down, but it really bothered me that he went that far. Instead of trusting me, he’s setting up traps like I’m guilty until proven innocent.

I don’t know if this is something I should try to work through with him or if it’s already a huge red flag. Is this insecurity something that can change with time, or is this just who he is?