It was established very early on into the story that Yuu has no place in his family anymore. He still lives with them, eats at the same table as them (maybe not so much anymore since he's always out in the streets when they're having dinner and whatnot), and sleeps under the same roof as them — but for all intents and purposes, he's a stranger to them by now. As sad as it might sound, he's almost like a permanent guest in their home at this point — just doing his own thing, not bothering them and not letting them bother him. Yuu himself said it that one time (paraphrasing here): "That gaze of pity... Even from my own family... I've seen them before". They just meekly and quietly accept that he's a failure now in the eyes of society, with no place among his peers at school or academic success to talk about. His true family might as well be his friends on the street by now. His blood family wants nothing to do with him anymore, and that's sad... But the thing is, many households are truly like this in our real world, even now.
They couldn't help him very well in his time of need when he was being severely bullied and ostracized, so he doesn't feel like he should even bother asking them for it anymore.