The guy's very ungrateful given that the girl gives him a chance at having a comfortable life while having to keep up with the constant stress of performing at the peak of her occupation. She never needed to take him with her.
He was not in a position of privilege to complain about her attitude when their lives hinged on her. Now that they're comfortable, he discovers the flaws in her character that were always there as shown in the flashback.
I doubt this manga will actually cover this dynamic and just focus on the protag power tripping as always.
I don't know how the WN goes, but as far as we know how things are going, we only got his perspective of the situation in a single chapter. However, the girl just dismissed his declaration of quitting the job and gave him more work to do for her, so it's likely she's been dismissive of his concerns and does indeed treats him poorly.
At a workplace, it's important for the other party to understand the workflow of their colleagues' work and the dangers and stress it entails, otherwise it becomes easy to just say "I'm the one carrying this company on my back" and throw your coworkers under the bus for your perceived flaws in their methods due to sheer ignorance. If she can't understand how dangerous the work of an assistant that gathers materials is, having to deal with week-long trips to dangerous places filled with giant worms and other mortal perils, it stands to reason she would think less of an assistant's job and refuse to address his valid complaints.
Yes, she gave him the opportunity to live a better life than poverty, and for everyone who doesn't understand what his workplace life is like, having to deal with mortal perils at every turn and then see his entire work be thrown in the trash without a second of hesitation, he may come across as ungrateful. But if life working under her is so hard that he considers abandoning the life of serving nobility to go back to poverty starting his life from scratch to be a better option, that says a lot more about her than it says about him.
It seems he understands her work and appreciates it, but she doesn't understand his and just dismisses him at every turn. We need more chapters to understand if that's truly the case, but so far that seems to be it.