What could cause childish behavior by a 26-year-old?

Hello doctor,

I have a 26-year-old elder brother who thinks like a child. He can be fooled easily. His male friends would call and speak to him femininely while requesting money. It escalated eventually, and he gave away my mom's gold jewelry without our knowledge. He believed he was in a relationship over the phone with a person he had never met. We tried to tell him he was talking to a guy, not a girl, but he would not believe us even when the person appeared to speak in a real voice. If someone tells him a made-up story, he readily accepts it, even when we tell him something is false. He is stubborn. Normally, people can tell right from wrong, but he is not one of them. He refuses to change his mind. It is difficult to advise him on what to do. We use that to our advantage to prevent him from being bullied. He has to be informed calmly and lovingly else, he will not listen. His so-called friends take his things, clothes, money, phone, and bike. Despite us telling him that they are misusing him, he does not believe us. I could guess that he is innocent and highly susceptible to manipulation. Little kids are more intelligent than him these days. He must be watched closely to prevent him from falling into the clutches of frauds who pose as his friends to steal from him. He was not at all bright in school. He failed so many times. While I graduated from high school, he was stuck in 11th grade. He cannot learn on his own. My dad used to sit with him and make him memorize so he would write something in the exam. Apart from this, he is living a regular life. It is just that there is something wrong with him intellectually. He has been like this forever. We do not want to alienate him from society and make him seem like a freak. Anyone who talks to him knows what he is really like, and unfortunately, there are not so many good people out there who would not misguide him. My dad does not want to see a psychiatrist because he is terrified, he will be on medication. I would like to know his condition so we can deal with it without feeling frustrated. I would appreciate it if you could explain his condition to me and, if you can, advise me on how to handle it. I cannot leave my brother alone, and I cannot leave my parents to take care of him forever. I am studying abroad, and I would like to make him at least a little bit wiser. Please help.

Thank you.