The man meets a human with Autism and is like
So he asks Lucifer to get him tested, and Lucifer is like "Okay sure"
And the results come back that he is, indeed, autistic.
This former angel, this demon, who by human standards is an ancient being of yore...
He gets diagnosed with Autism. High-functioning Autism, but he's autistic regardless.
Boy suddenly feels way less alone in the world because he knows there are other people out there who hyperfocus on things they love just like him, who can't socialize or easily relate to other people just like him.
I also feel like after having this revelation, his brothers would ease up a bit on the teasing.
Brothers react (quickest acceptance to slowest)
-Surprisingly (other than Luci and Satan, because...c'mon, Satan loves knowledge and Luci is a Responsible Big Brother *tm*) Mammon is the first to learn more about autism. He may not be the smartest of the brothers, but (in my opinion) is probably the one that's most in touch with his feelings and his empathy, despite still having that selfish streak.
-He wants to know more about Autism so he can understand Levi better and help make his brother more comfortable during their interactions.
-He'd also start stealing things from Levi less, because he now knows that there's another level to that memorabilia from anime and games for Levi than just stuff he likes.
-Will 100% buy Levi a stim toy when he gets some extra cash but shhHHHHH it's not from the tsundere crow man
-Probably comes around to fully understanding everything the quickest, despite both Luci and Satan reading up on it n stuff.
-This one is tricky, because he understands what autism is from a logical standpoint, but he still has a bit of trouble really understanding how it effects Levi
-Will also get a stim toy for his brother, probably like an infinity cube or something
-Still not really getting it until Levi has a meltdown
-Lucifer has dealt with Levi's meltdowns before, but moreso in a "my brother who is the avatar of envy is just throwing a fit" way instead of a "my brother who is having a meltdown as a cry for help needs me" way
-Instantly goes to Levi with probably either a handheld game or one of his stim toys
-Uses nonverbal ways for his brother to communicate with him so he'll know how to help him through this
-Will 100% get his brother noise canceling headphones and also take rigorous notes on what is ok with Levi and what isn't when it comes to stimulus
-Autism??? What's that???
-Poor boy is a bit dense at times but we still love him
-Supportive but not really sure of what to do or what to say or how to react
-If Levi had a meltdown around him though you'd best be certain that he'll be the first to scoop his brother up and REMOVE HIM FROM WHATEVER SITUATION IS CAUSING HIM HARM
-Gradually starts to understand more and more about the condition
-After all, his brother is still his brother. This label doesn't change anything, Levi has always been autistic, this label just helps people better understand him
-Will be the first to deck someone if they make fun of Levi for his autism. Doesn't even care if he gets in trouble later, you don't make fun of anyone, especially his family, for something they can't control.
-OK I know I put Beel higher than him and this is why
-Satan read up on what Autism is, yes.
-But the man is still TECHNICALLY the youngest. He is the only member of the family who was never an Angel, as he was born from Lucifer's wrath toward everything when Lilith "died", which was when Lucifer ceased to be an angel completely. Everyone else was an angel at some point, so even though Belphie is the "youngest" because he has the weakest power, Satan is far younger than everyone else when it comes to actual age. Okay? Okay.
-As the youngest (and I am technically correct here on him being the youngest ok bye-) and because he was literally born from nothing but wrath and hatred, the man is still learning on all the other emotions. And this is a canonical fact, anyways.
-So like his brother-father, Luci, Satan understands what Autism is, but he also doesn't understand what it is at the same time.
-He read up on it, that's good enough for him.
-He still cares about Levi, and just like everyone else (even though I only listed it for Beel) he knows that this label doesn't change who Levi is at all
-Gradually he starts getting a better grasp on everything. Will 100% be next in line to deck someone if they make fun of Levi for being autistic.
-They both like TSL so it would be an activity where he could really relate to his big brother and learn more about how his brain works so he would be better equipped to help him and how to best be there for him and relate to him in the future.
-Belphie is kind of a jerk toward everyone except Beel. But it is clear that he loves his family in his own weird way (even Lucifer, but he'll kill you if you even think of revealing this fact to Luci).
-Doesn't really get it, but the man isn't as self-absorbed as Asmo, which is one main reason I think he'd come around to it quicker than the lustful demon.
-Does want to understand it, but is also asleep like 70% of the time, so when anybody tries explaining it to him he ends up falling asleep.
-Eventually will probably get an audio book about the subject so if he falls asleep he can just resume from wherever he remembers it leaving off.
-Understands how Beel acts and reacts to situations where Levi is having a meltdown or is getting overwhelmed by noises or amounts of people and attention, so this does for sure help him better understand and accept the condition. Because Levi is still Levi, ultimately.
-Will probably let Levi rant to him about his favorite things. It would make Levi happy and seeing his brother happy would make him happy, even if he ends up falling asleep during Levi infodumping onto him.
-Not that he doesn't wanna get it, but he was barely paying attention when the results were shared with everybody because he was looking at a makeup tutorial on DevilGram.
-Like a month later he's pulled aside because he did something to really upset Levi, like bugging him repeatedly to take pictures for the gram with him and forcing him to livestream, and he's like
-"Yeah but I always do that?...he had a meltdown? What's a meltdown?"
-Will actually look up what a meltdown is after this
-Probably feels terrible but still doesn't really get it.
-"Why should I have to change myself for him?"
-Because you shouldn't be consciously making anyone uncomfortable, autistic or not, ASMO.
-Probably takes the two of them getting into a big fight and Levi to actually start crying for him to snap to his senses
-Will actually take time to do research and try to understand it better
-Still doesn't really fully get it, but he's doing his best to do this for his big bro