All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
Indeed, you did well by seeking what is lawful and ascertaining the Islamic ruling before taking action; indeed, this is how a pious wife should be, keeping her husband away from what is forbidden, so may Allaah reward you and enable you and us to do what pleases Him.
As regards your question, the answer is that it is not permissible for a Muslim to subscribe to commercial insurance unless he is forced to do so. You mentioned that insurance is not mandatory (in your country) except for traffic insurance; therefore, it is not permissible for you to register your husband in trade unions that insure its participants with forbidden insurance companies unless the subscription to this union is for another permissible benefit, like protecting the worker’s rights and defending him if he is wronged and so forth, and provided that the fees are paid for this purpose and not for the purpose of insurance, in which case, it is permissible to register your husband in that trade union and pay the fees for the subscription for that benefit.
Allaah Knows best.