Being a developer always means getting stuck on some trivial crap for a full day and going to bed angry. One simply makes up for it by those times when you create fantastic value in thirty minutes. You find the bug right away, or a new feature or deployment just falls into place smoothly. Software development is like solving math problems - it's not a continuously progressing process, no matter how much the agile priests want it to be.

This developer sounds humble. That's the most important quality a developer can have. If one becomes too humble (impostor syndrome) then one needs an environment of more encouragement. That's the job of peers and managers.

There are lots of bad developers. It's not unthinkable that this guy is a bad developer - but it's still the managers problem. Is he trained, mentored and encouraged enough?