It turns back to what I said before, the lunch box that he sees as "luxury" will be the least of his problems if he gets hurt or too sick, because hospital and medication bills will be more expensive than the lunchbox, and if he expends too much on medical bills he can't help his mom.
Is a chain: gets hurt —> can't work —> no work —> no money —> with no money —> uses savings to afford bills —> savings partially gone —> sends less money to mom.
My guess is that maybe he's more mentally unstable than what we have seen, or the author just didn't bother to think about it, according to people that have read ahead, the story doesn't make much sense anyways.
You said so yourself "some people actually do live not for themselves, but for their love ones" this is what I'm referring to as well on my hypothetical discussion, how are you going to help your loved one financially (which is this case) when you have no money because you felt yourself about to faint but instead of eating you fainted causing medical bills and hours not paid due to fainting? Which one is more expensive, a dingy sandwich or a medical bill? With what money are you going to help if you're living and helping paycheck to paycheck?Understand now?
He already went through this when he had the accident, so why is he acting like he doesn't know the consequences? This is why I reached the conclusion that the author didn't really paid to much mind to that and according to readers that read the raws the story is lacking some sense later on, so I'm just going to throw all sense out of the window and move on.
You're making sense but that's not what I meant by that at all. Like living for other people does not just mean financially helping them or helping them in other ways, I mean like constantly looking for approval of those around you(eg, his mom and that ML guy smth). If that clarifies it. He honestly looks like he is ready to die until he developed some type of validation from ML. Plus I'm only talking about this part of story to share my perspective, it's understandable if you think the later on parts are poorly-written.

I don't understand why didn't he just buy the food, you waste more money in medications or the hospital if you pass out and get hurt (trust me on that one) and if he's paid hourly, hour that you don't work, hour that won't be paid, which he knows first hand because when he had the accident he had to stop working and that worsened his financial situation, do you not learn from what you've been through??????
I understand there are other expenses as well, but he will eat at the end of the day anyways. Maybe he didn't want to waste money because he has food at home, in which case why not just prepare your own lunch for work? Not like we know any details since the author hasn't explained almost anything about him, does he study? Does he has hobbies? how are his living conditions? Does he has any friends or acquaintances besides that one person that recommended him that job?