Hmm. I'm trying to look at this as objectively as I can, since I don't care much for being dragged into feeling one way or another for the MC. So the nobility has a duty to enter the labyrinth in some way shape or form, likely something along the line of having to protect the people, so likely nobles are in effect capable fighters in some way shape or form, or more to the point drafted into the army as a point of comparison. So it's hard to tell but the scribe brother may either be an actually capable fighter, or just handled a support role when tackling the labyrinth.
The MC is clearly overly coddled. He's fat, out of shape, sentimental which that's fine he's a teen/kid/whatever. But also has no cares about his duties to the point where he never inquired upon the duty to the labyrinth that he has as a noble. He also has no proper pragmatic sense either. The items could have been taken and bartered afterwards and at worst, having a dagger as a secondary weapon and as a general tool would be far more beneficial for him, especially since he would be going out there with seemingly little to no money outside of whatever pocket change he had accumulated. Which only goes to show how sheltered a life he has had which has allowed him to become such a fat guy in the first place.
And he wasn't disowned in terms of no longer being nobility, but being shamed by his father. He is exiled from the house until he completes his noble duty, but that doesn't mean he suddenly loses his title or authority as a noble, so he's still just going on the sentimental route by proclaiming he's no longer a noble but an adventurer.
Also it doesn't sound like his father abandoned him as he grew up. This seems more like a case of miscommunication. Plus it was clearly that he did not properly understand the underlying rules of nobility. If he was given an education, then I am going to assume that he would by nature also have a teacher to teach him etiquette. But he seemed to be single minded about just proving his capabilities for his personal prowess and that got him to get blinders on.
And him proclaiming he has little money left while we see a bag full of coins does not provide good context. For all we know these are all gold coins and they would actually last him a long time, but if he has no knowledge of the value of money and that's despite him stating he has dedicated so much time in studying, it bears into question how much he understands about finances, which again would be part of his noble duties. So on top of being ignorant about the labyrinth that is part of his noble duty, he might also be completely ignorant about finances, which in turn just continues to indicate he was pretty much a self aggrandizing worthless individual.
Honestly, it sounds like he did all the wrong things up to this point and he's going to be magically rewarded for being a mess and a worthless being. He even knowingly goes into what was clearly indicated to him as being dangerous and life threatening head on, knowing that he is out of shape and having clearly lost his ability to wield the blade as good as he used to if he could not even attempt to fight back against a goblin.
I get that the intent is to make him sympathetic. But I can't help but think he's just someone playing victim despite literally being in the lap of luxury and ignoring his duties because he just wanted to show off how strong and smurt he was without ever actually learning a damn thing.
Maybe the novel did a better job portraying the issues in his family. Or maybe the intent truly is to start you off with a useless, naive protagonist and he will actually correct himself through hard work.