It appears to be a case of Schizophrenia for which there are injections which give long term results apart from the conventional tablets.
Next Steps
Visit the nearest Psychiatric centre for active advice and admission protocols as needed and have regular follow-ups.
Health Tips
Try not to convince him because it's possible he lacks insight that he needs help. Act on his behalf. Consult a Psychiatrist. You have the right to do so as his gaurdian. Your permission and understanding is necessary, and is more than enough for him to get the treatment he needs.
I hope this helps.