My brother changed a lot after Covid and I don’t know how to help him

Assalamu alaykum. I need to be honest - I’m talking about my own brother. He’s in his twenties and after Covid he’s become a completely different person. This isn’t just a short phase; it’s been his way of life for years now. He’s become extremely strict in his practice and constantly argues with family members who have studied Islam their whole lives. He seems to trust strangers online more than our parents, and real conversation with him is almost impossible. His habits and appearance have changed a lot too. He gained a lot of weight, stopped taking care of himself, and isolates from people his age. He mostly spends time with much older men who already have families. He refuses doctors and medicine. When he’s ill he says religion forbids taking medication and that olive oil is enough. We even suggested seeing a Muslim psychologist, but he won’t accept help coming from family. All of us - my parents and siblings - have tried to talk to him seriously. Each time he promises to change, then doesn’t follow through. When we confront him he avoids the subject, changes the topic, says “astaghfirullah,” or just smiles. Now he wants to get married, and that scares me given how unstable his life seems. I’m not mocking Islam; I’m describing real changes that worry me deeply. I’m mainly worried for him and even more for my parents. Has anyone here dealt with something similar or found a way to help a family member in this situation?