Is it permissible to keep the IUD to prevent pregnancy, knowing that it is unlawful, and my husband refuses to remove it due to his old age and inability to raise more children?

Procreation is a shared right between spouses, and neither of them is permitted to abstain from it without the other's consent, except for a valid excuse. If the spouses agree to forego procreation for a legitimate reason, there is no sin in that. There is no objection to removing an IUD if the agreement between the spouses is to avoid procreation for a valid reason. It is permissible to use it for a need or to ward off harm.