Iβve always viewed him as being autistic. I doubt he wouldβve been evaluated/diagnosed as a kid, and as an adult (in the late 80s/early 90s no less), heβs certainly not seeking out that diagnosis. Iβd say heβs what we now call low support needs, high functioning in outdated terms. Maybe Iβm just projecting, but heβs always struck me as neurodivergent.
Heβs definitely anxious, though in the context of the show, thereβs never a solid moment of reprieve for him. The anxiety when heβs not in the middle of danger shows itself (I think) best in season 4, when he and Argyle are playing junkyard golf. Heβs talking about Nancy and starts talking about how their future kids will hate him like heβs so sure of it, that these theoretical kids that donβt exist already despise him. Heβs high strung, but again, context is so important.
He was set up from a young age to shoulder a lot of weight. By the time season 1 starts, heβs already working, he took an extra shift in the first episode and that money goes to bills.
As far as the undiagnosed autism goes, he avoids crowds, we see him avoiding the cafeteria in the first two seasons, leaving while Nancy and Steve are eating lunch in season 1, and outside the school to eat in season 2. I imagine he gets overwhelmed in loud environments, especially if heβs already uncomfortable. I also see him as fairly touch averse (while also being touch starved, a fun /s combination), with the only real exceptions being Joyce, Will, and Nancy after a little while.
Chronic pain post season 3 (I missed the last discussion, but heβs certainly got back pain and likely ongoing headaches after the hospital fight) doesnβt help with those problems. In Lenora, he seems to only find support through Argyle. His family, Will, Jane, and Joyce, are all dealing with the changes too, and Jonathan isnβt someone to bring up his own struggles, but Argyle is someone he finds himself leaning on. Eventually literally, the two of them becoming close enough that Jonathan can find comfort in touch with him. Argyle listens and asks questions and is never cruel about anything, one of the first genuine friends Jonathan has ever had that isnβt his little brother or girlfriend.
I also imagine he has difficulty with emotional regulation. Heβs quick to frustration and anger and can lash out because of it. When the world isnβt ending and he has just a little more space to breathe, he tries to shut it down rather than feel it. When he knows something will get him worked up, heβll just avoid it entirely. Including reflecting on himself and his relationships. Heβd rather let them stagnate than let himself get upset in a conversation. Maybe because he sees parts of his father (or even Joyce) in that anger and doesnβt want to turn into him. Doesnβt want to be an angry person. So he represses and pushes things down.
Music is a big source of comfort for him (which is why I think he shouldβve had a bigger role at the radio station in season 5. Of all my problems with season 5, I think it was a huge missed opportunity to not have him assisting Robin with broadcasting. He probably wouldnβt have wanted to be the one talking, but he couldβve been in the booth! He shouldβve been her assistant instead, sorry Steve!). We see him using it to connect with Will, going to it when Will is missing, and he has a well loved collection of tapes and records. (And then heβs just not there for the music-heavy plot in season 4, another missed opportunity!!!!!)
Anyway, this is getting very long. But my last point is that Jonathan feels like an outsider, and maybe thatβs because he is one. Being autistic can be isolating, and heβs already got enough going against him in a town like Hawkins. Struggling to connect with his peers, to understand his peers, goes hand in hand with it. He might think heβs doomed to his loneliness, to isolation, and might even posture like he wants it that way, but itβs hard. Heβs not alone though. By the end of season 5, heβs shown to be doing well. Out of Hawkins, finding new passion in filmmaking, friends with the other three young adults (Robin is likely also autistic, though like Jonathan, likely doesnβt have any formal diagnosis. Regardless, I think they might find kinship in each other without fully recognizing thatβs why). I wish it had been explored more, that we couldβve seen more in depth moments with his character, more moments that expanded on his relationships outside of Will and Nancy in canon. But thatβs what fic is for, right lol