What is the duty of parents towards their 18.5-year-old son who steals money, utensils, and clothes from them and sells them, and has been committing these acts for 9 years?

If you are able, advise your brother that what he is doing is impermissible. Otherwise, seek help from someone who can advise him, or ask the Imam of the mosque to address the topic of theft in his lessons. If he repents and returns what he stole, then all praise be to Allah. Otherwise, inform your parents so they can work on disciplining and reforming him, as Allah Almighty says: "O you who have believed, protect yourselves and your families from a Fire whose fuel is people and stones, over which are angels, harsh and severe."