Concerned my father may have strayed from Islam

Assalamu alaikum, everyone. I recently found out something troubling about my dad that makes me fear he might not be Muslim anymore, and I'm really worried about how to help him. My father hasn’t prayed or read the Quran in nearly 20 years. He sometimes fasts, but only to support me and my sister when we’re fasting in Ramadan-he says he does it out of love for us as his daughters. He doesn’t believe in the Day of Judgment, and just now I learned he thinks Prophet Muhammad (peace be upon him) wrote the Quran. He also has some belief in astrology. I’m afraid all this could mean he’s left Islam. He still believes in God, but not these essential parts. He didn’t tell me this directly; my mother shared it with me, and I got very anxious. Whenever I gently tried to encourage him before, he’d say it’s a matter between him and Allah. It pains me to think he might not be Muslim. He doesn’t openly say he’s not Muslim, and he himself doesn’t think he is. He just disagrees with core Islamic beliefs. I’m also worried about my mother and sisters-we are all practicing. But if my father has left Islam, wouldn’t that nullify their marriage? My mother doesn’t know, and I don’t plan to tell her now. She just thinks his faith is weak. I don’t want to turn our lives upside down, but I do want to guide my father back. I intend no disrespect; I know only Allah grants guidance. What should I do? Even if I told my parents their marriage is invalid, they’d never believe me or act on it. Does it become void immediately, or is there a waiting period (iddah)? Sometimes I clear up misunderstandings about Islam for non-Muslims online, so I know how to make convincing points. He’s had a tough life, and I think he says these things because he feels Allah doesn’t respond to his du'as-he basically believes Allah no longer cares for him. He’s still a good person. I don’t want this to destroy our family, but I’m confused. I can’t reach a local imam, and online scholar question portals haven’t replied. If my mother must observe an iddah, time could be limited. It feels unreal, like nothing will happen, but I know it will, and I can’t ignore this. Please pray for his guidance and advise me.