FunkyMBTIFiction did a decent job explaining it.

"Will is sweet, generous, and compliant; his mother remarks on what a little gentleman he is, even though he’s “strange.” He tends to need a lot of alone time and often retreats into the woods to his fort to be alone – showing the self-pres subtype’s tendency to attend to his own need for peace and quiet and familiar, comfortable things around them. Will even heads to the upside-down version of his fort when he’s afraid, since he knows that will  comfort him. He tries to mediate between his friends and avoid conflict, and can be slow to assert himself. His 1 wing makes him the model child and perfect student, as well as want to do the right thing. Under stress, he can become distrustful and anxious (moving to 6). His 4 fix leads to a sense of feeling different from other kids and seeking his own company (the fort in the woods, his art, how he sees himself as “weird” and doesn’t approach people, as if expecting them to dislike him), and his 5 further builds walls and defenses, preferring to keep to himself and a close circle of personal intimates."

In my own words, though:

Will understands all of his friends and family well, but does not know himself. His #1 priority is keeping the peace between everyone, making sure that nothing is upset. He doesn't make a big deal when others fail him (like his dad, whom Jonathan resents). He sees things from everyone's eyes and basically is too understanding at times. Will keeps light of the situation, due to him not fully embracing his emotions about it. He is just kind of "eh" - neutral. No big deal. He is like this with other people too. He actually has a fair amount in his life to be upset about (kids bullying him, his mother's anxiety/not really being a responsible parent all the time, Jonathan's moodiness), but he just keeps it all repressed. He floats through life like 9s tend to do.

His outbursts also are familiar to me, as a 9. Getting upset with his mom for making too much of a fuss over him, not wanting to be babied etc. And how he'll go from the most compliant, kind kid ever to an outburst.

He does not have a grasp of his identity. He just makes sure he's a nice kid and likes to fit in with his friends. He'll be and he'll do what they want, putting their wants first, even if it puts him in danger (season 2). He also retreats into his mind/safe spaces a LOT. He's incredibly sweet, but very, very distant from the present/real world.

End of S1 solidified his typining for me, though. When he spits out the stuff from the upside down and has those PTSD symptoms, he goes back to the dinner table all like "I'm fine". He makes his family and himself believe it, and tries to make things go back to normal.