Hi Doctor, I Have An Elder Brother(26 Years) Who Thinks

Hi doctor,
I have an elder brother(26 years) who thinks like a child. This is not a case of
Man child. People fool him easily. Some of his guy friends used to call him and talk to him in a girls voice and ask him for money. Eventually it escalated and he gave away my mom's gold jewellery without our knowledge. He thought he had a relationship with someone on the phone that he never met. We tried to tell him that he's talking to a guy not a girl,but he wouldn't believe us. Even when the guy kind of actually talked in his real voice. If people tell him a fabricated stories like this he believes them easily even though we tell him it's not true. He's really stubborn. Normally people can distinguish b/w wright and wrong but he can't. He sticks to what he's told. And he has to be told in in a certain way and it's very easy. He'll stick to a belief no matter what.
We use that to our own advantage to prevent him from being bullied.He has to be told calmly and and lovingly otherwise he won't listen. His so called friends take his things,clothes, money, his phone even his bike. Despite us telling him that they are misusing him he doesn't believe us. What I know from all this is he's really innocent and can be manipulated very easily. Even little kids are smarter than him these days. He has to be constantly monitored otherwise he will stray into the hands of people who pretend to be his friends and take money from him. He was not at all bright in school. In fact he failed so many times that while I graduated from high school he was still stuck in 11th grade. He cannot learn on his own. My dad used to sit with him and make him memorize again and again so at least he would write "something" in exam. Apart from this he is living him a normal life. It's just that there's something wrong with him intellectually. He has been like this forever. We don't want to alienate him from society and make him seem like a freak. Anyone who talks to him knows what he's really like and unfortunately there aren't so many nice people out there who wouldn't misguide him We don't know what his is. My dad doesn't want to see a psychiatrist because he's afraid he'll be on meds.I would at least like to know what his condition is so we can deal with it without feeling so frustrated all the time. I would be really grateful if you could help me understand his condition and if possible tell me how to deal with it. I can't leave my brother on his own and I can't leave my parents to take care of him forever. I'm studying abroad and I would would like to make at least be a little bit more wise. Sorry for any grammatical mistakes English is not my first language.