A little rundown on the AU: Jason comes legally back from the dead after Roy uses some of his old contacts, they get married to ensure power of attorney, Jason changes his name to Harper both because he was feeling mushy about it and because it's easier not to immediately have press hounding them because no one has heard of Jason Harper before. Jason has been recovering from his injuries for a while now and while he is figuring out what he wants to do on a vigilante side of things, he figures out his civilian life. He gets his high school diploma, gets an actual driver's licence, things like that.
Then he wants to get a job. He starts as a regular mechanic because he knows how vehicles work. It's fine. He gets a couple of certifications. He then just. Keeps going. He gets so many mechanic certifications for different vehicles because he already knows a lot and has always been a fast learner. He eventually ends up working as an aviation mechanic for a private hangar and he gets a very good reputation with their clients because no matter what type of plane they have, he knows what's up. He also knows what's up with their cars. Their bikes. Their boats. Their helicopters? One of the clients is an army veteran. Jason knows all the stats on all of the combat vehicles the guy ever mentions.
Jason's boss and coworkers have seen all the certificates. They are all framed on the walls. They all think that Jason very clearly has either autism or adhd or both and hyperfixates on vehicles. They are all extremely supportive of his needs and will listen to him talk about books because one of them read that it's important to support their interests. They keep calling him Sheldon and making The Big Bang Theory references. Jason has no idea what they are talking about.