I think being a good listener can help too. Keeping the lines of communication open and free from unpleasant reactions to his concerns. I think having doubts and questioning about this gospel is universal. if not essential to the finding and securing of lasting testimony. Perhaps if he felt that he had a safe place to discuss his feelings and questioning. Maybe he needs to explore the truth and even argue with it. Wouldn't it be great if you could do that together without argument or fear of mom getting upset or overreacting? Wouldn't it be great if he could express some feeling and you could ask him questions about things he hasn't thought of before or vision questions that lead him to his own conscience and his own responsibility? Perhaps then the Spirit can direct you to speak of truth or perhaps quiet you when he needs to find something on his own.
I wonder also if a negotiation might be good. Perhaps you could create an environment where he is free to search and question but only under certain circumstances. Maybe you say, "i understand you are questioning and exploring right now....which is fine....But I would like you to continue to attend at least this one meeting or teach FHE once a month....then you can choose what you do with A, B, and C etc." Then perhaps he can learn how to explore his agency AND obey his parents at the same time. I don't know. I am kinda pondering aloud here but he is still a minor and still is subject to the rules of wise parents. I don't think forcing is the answer but I don't know if leaving the parameters to his choices open is wise either. Maybe you determine a comfortable and flexible parameter for his searchings.
I don't know. Does he want to read about other faiths or explore different perspectives of thought? Perhaps you can direct and supportively participate in such pursuits. Maybe one FHE could be about the different faiths of the world. Or maybe one about how we determine truth and what the spirit does on either side of obedience. And then maybe he needs a safe place to make a mistake or two....which may be cool cause then you hopefully can process the mistake together. Sometimes our mistakes are the best teachers.
I think too, that you should act from a place of love and faith rather than fear and panic. ( Or maybe just take your fear and panic away from his view.

) I believe that God will help you discern his heart and determine his needs and then help you with a plan to meet those needs in wisdom.
Good luck, my dear. Man, this just isn't an easy one.