In the medieval era Europe such people were sent to the clergy. Genius-level intellect is fairly obvious even in children as it involves thinking different from normal and children are bad at hiding that. As such the kid would have ended up talking to the local priest or the local monks unless some specific reason prevented it. This would have led into him being educated to some degree and probably being forwarded elsewhere for further education, which was pretty much a monopoly of the church. Even if he dodged education religion was seen as only proper intellectual pursuit so expectation would have been to go there.
You mention being sheltered, so I think the assumption was he has spent his childhood with someone who protected him from falling to religion and getting educated? Developing that person and relationship should probably be your first step in creating the character as his values and attitudes would come from there. Reclusive relative, wandering magician, minstrel, or merchant? Somebody who lives separate from others or moves around, anyway?
As for writing a genius-level character, you probably should not. It is almost impossible for an author who is himself not at that level of intellect to do it correctly. And while most people would not notice the issues, they also would get no added value over the character simply being very intelligent with 120-140 IQ. It can still be impressive and it is much easier to write. Much easier.
Some tips if you really want to anyway. These should help as long as you avoid the genius intellect being the focus of the story. And you really, really should avoid that.
"Normal people think inside a box of their expectations. Intelligent people can with effort think outside the box. A genius builds a fake box and uses it to communicate with others."
Genius has issues communicating with other people. They are constantly aware they are different and separate from other people. This includes their family and friends. This may result in withdrawal and bad social skills or it may result in deliberate effort to act normal. This results in a person that most of time acts perfectly normal, but when excited or upset says incomprehensible or odd things that display, if somebody cares, greater understanding.
Genius always redefines the issue at hand. Genius does not really have a real copy of whatever box people are using to frame their question. So they have to reframe it in wider context to understand it at all. Upside is that this usually gives deeper and wider understanding and possibly "outside the box" solutions. Downside is that unless the topic is already familiar, the genius needs more time to have any opinion. Usually being "merely" very intelligent is more useful than being a genius.
(<- The part to remember whenever faking genius thinking.)
Genius thinks other people are smarter than they are. Because genius reinterprets everything in a wider context, he sees links and meanings that did not actually occur to the person saying it. Usually this is harmless, but can lead to misunderstandings. Essentially a genius will assume people are expressing their thoughts in simplified form for easier understanding,because that is what they themselves do.
Genius excels and totally sucks at explaining things. Genius may see the issue with great clarity and consequently can provide a clear and accurate explanation of it. Genius is almost incapable of predicting what of and how normal people will understand his explanation. No copy of the box, remember? This may result in incomprehensible and easily misunderstood explanations.
Genius is bad at explaining what they are thinking and usually will just say something "close enough" if asked. While the end results may have great clarity, the actual thought process may be non-linear and impossible to verbalise without extensive ret-conning. This is a limitation of languages and verbal expression and not specific to geniuses, but a genius will run into it constantly. Amplified by most geniuses also being very intelligent and thinking faster than they can talk.
And a reminder, IQ was originally seen as the quotient between mental and physical age. So making the character prefer talking with older people and express himself as an older person would, with allowances for social status and life experience, is a good start.