Suwoo, I read your previous thread but didn't comment as thought you had had enough advice, but I feel too strongly to stay silent now. Tiktok I think is getting at this, but is too balanced and professional to lay it on the line. Although that may be an assumption.
Your baby is not gaining weight. He should be. This means he is not getting enough calories. This means you have to get more into him. Since you've done so much to try to do this by breast feeding directly, you now have to try something else, which means supplementing either with EBM or with formula, via an SNS, cup feeding or bottle. Your choice, but you must do it as he is now, as you describe, looking like a skinned rabbit. This I'm sure you are aware is not what babies look like, even those who will turn out to be naturally skinny children.
It is more important that he starts growing than that you persevere with from-the-breast feeding right now. Later on, as others have described, you may be able to get him back on the breast.
Could you do one big express in the evenings when all your dc are in bed or asleep? power pumping can help boost supply - do 10 mins pump, 10 mins off etc for as long as you can stand. If he's going for a long break at night then wake up and pump while he's sleeping, then you won't feel so bad about giving some formula.
I know this is really hard, I experienced it with my dd. I felt awful about her stalling growth, and awful about having to top her up, sometimes with formula, but it is what it is. She too seemed to feed well, sleep well, pee and poo etc. But she wasn't gaining and that was a problem.
Sorry to be so blunt but I really couldn't bear it any more!
I hope things get easier.