Dong=gyun isn't a child, why would he celebrate his birthday outlandishly?
Dong-gyun is living by himself, his family are likely in another part of Korea, and he's an adult, he's not going to call them up wanting them to rush over because he has a cold.
Dong-gyun hasn't eaten in expensive restaurants... so what, he likes BBQ and snacks
None of this suggest anything interesting about his background.
Most adults still celebrate their birthdays with something, even if it's just a small cake. He could have celebrated with Chanwoo or Jaejin but he obviously doesn't feel close enough to them for that. Or maybe he didn't even want to celebrate it, and there's generally a reason for that too. He said he felt lonely because he had no one to call to help him aside from Chanwoo, even though he technically also had Jaejin, the one who got him sick, guess he really doesn't feel close to him. Even middle class families will sometimes take their child for a birthday or a night out at an expensive restaurant, so this could indicate whether he was poor growing up or if his family didn't celebrate lavishly or if they just plain didn't celebrate birthdays. Not all of the points have to mean something, I just picked out the stuff that the author seemed to be highlighting, since otherwise, it'd kind of be useless information. Being alone seems to be Donggyun's overall theme, so is it totally out of balance to say we'll get a reason why from his background later? We got an explanation for Jiwon's actions, we should also get one for Donggyun's actions, since he's the other half of the story and a lot of his actions don't make sense to most people who have even just a shred of dignity.
would have to disagree, as said DG's lifestyle will be different from when he was a child. He is living alone, he has other priorities, different financial situation etc. DG going to uni & work is probably more representative of how most people over 18 celebate birthdays than having a specific big 'date/party/anniversary" to mark it, unless it's a special number like 60 then a small family meal may do. We don't know if DG's parents phoned him to wish him well that day, or if they sent some gifts in the post, but even gift giving over 18 isn't always the norm. It actually comes off weirder, and more KDrama that Dong-gyun is having a massive post-birthday date where Jiwon spends a lot of money on him.
Jiwon is being financially supported by his parents. He has more disposable income than DG, and we know he is a snob given his comments about the BBQ. DG preferring to eat at BBQ's/cafes is more of the norm.
DG is living independently at university, why would he call his parents up about every cold and sniffle for them to travel over to him? The only person he can call is Chanwoo, to go pick up meds. He hasn't made any other connections. Also, this is yaoi where 'character gets a fever' is just a tedious dramatic cliche.
The author seems like they are going for that stupid kdrama rich x poor trope, and slipping in shallow stuff like the points you noted, as if it actually is important.
It's fine if you disagree, as I said, it's just my theory. And my norm has it such that most adults, including Korean adults (I lived in Korea for a year as an exchange student), celebrate their birthdays, even if it's just something small, as I said before. Maybe it's the norm wherever you are. Why do you keep bringing up his family regarding the cold? I never said that he should've called up family, nor do I think that. Most people have a few people they can call up for help, even if it's just an acquaintance, for getting meds and stuff. I was an Indian kid in Korea, so it was a bit difficult for me to make friends, but even then, I had several Korean acquaintances outside the exchange program who came and helped when I got a bad flu (of course, now they're my friends). Donggyun getting a cold was a cliche, but why even insert of a scene of him saying he was lonely at all, when Chanwoo was helping him? That has nothing to do with being sick and Jiwon coming over. Again, you can disagree, I'm just pointing things out.
Also, I see no better way to make room for something like Donggyun's background than to make the obstacle come from Donggyun's side. Last time, the obstacle was Jiwon's hatred of love, which is cliche, I know. If the problem comes from Jiwon's side again or if there's no problem at all and this is the end, then the background on Donggyun as promised by the author would have no real sensible place at all.
I honestly think the next obstacle will come from Donggyun's side. I've noticed that the author seems to be slowly building things up about his background and what his life was like before BJ Alex. Like how he's never loved anyone before, how he didn't celebrate his birthday and thought that was normal, that he's never eaten at super expensive restaurants, even for his birthday, that he played candy stall games as a kid, etc. The biggest thing I consider a flag though is that when Donggyun was sick, he said he felt really lonely, similar to how Jiwon noticed that Donggyun is always alone. I think pretty soon we're going to see Donggyun's background and something from his background will serve as the next block in Jiwon and Donggyun's relationship. If this is the case, I think we're in for an interesting and possibly final arc.