It was mentioned earlier that he most likely didn't level up since the monsters were supposedly illusions made by magic stone as opposed to real ones. Defeating real monsters is pretty much the only way to lvl up.
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He does eventually start going up in lvls. If I remember correctly, he cries when he gets to lvl 2.
I read the manga version which is available at the site below. It is in Japanese though.
https://rawdevart.com/comic/seija-musou/
You could also check out translation for the light novel.
https://www.novelupdates.com/series/invincible-saint-salaryman-the-path-i-walk-to-survive-in-this-other-world/
This is so good but I'm curious about the main priest person and about the labyrinth and why it's a secret to the adventure guild. Also why the monsters don't make the mc level up. Sus...