@dethduck.
3 reasons:
1. He had a huge debt to pay up left behind by his parents who are now making a trip around the world or so.
2. Onee-san has enough money and influence to easily pay up that huge debt.
3. The parents transfered the rights of legal guardianship over to Onee-san cause she payed their debts.
So he has no money and Onee-san has enough money, connections and the legal rights to find him and take him back or move next to him again anyway.
Hence he cannot escape even if he wished for it.