@BuffSoul : You might want to consider that OVER A DECADE OF CONSTANT ABUSE AND TORMENT will strongly affect someones self image. That negative self image does not disappear over night. Not a chance, only an idiot would think otherwise.
Plus it was more then just being overweight, EVERY aspect of his appearance was horrible, especially his face, and he can not tell it is improved without a mirror.
As to school, he is worried about not being a productive member of society, something that is strongly ingrained into the social pressure of the Japanese. Not being a NEET is basically the only positive part of his life at the start.
That and you need high-school to make pretty much anything with your life, especially in Asia. Yes he can use the funds from the otherworldly, but that WILL cost him in the long run, since he will get accused of tax evasion and lose his grandfather's house as a result