What is the ruling of Sharia on depositing the money of a sick son in a foreign bank to benefit from its interest, knowing that they live in a non-Muslim country and his social welfare salary is not sufficient?

It is not permissible to deposit money in a usurious bank for the purpose of investment. As for depositing it for safekeeping, it is not permissible except out of necessity or a need that is considered equivalent to necessity, such as fear of theft or the need for transfer in the absence of an Islamic institution that can safeguard the money. Similarly, it is not permissible for your son to benefit from the usurious returns except out of necessity.