Is it permissible for a son to buy his father’s house from a usurious bank to avoid the family becoming homeless, knowing that the father owns nothing else, and that the son detests dealing with usury?

If the father mortgaged the house for his original debt, and not for its usurious interests, and the bank had the right to sell it upon the father's inability to pay, then there is no harm for the son in buying the house from the bank, especially if the intention is to protect the family from dispersion. It is permissible to deal with usurious banks in permissible transactions if there is a need, just as the Prophet, peace and blessings be upon him, dealt with the Jews. The father must repent from dealing with usury before it is too late.