“Only Ms. Rosa's boy, running around in his Halloween costume, more than a week early. I swear, the nerve of that woman leaving her brat unsupervised.”— Codsworth mistaking the ghoulish appearance of Rosa's son for a costume.
Before the War, Ms. Rosa's son was presumably one of the neighbors of the Sole Survivor who was restoring an old Corvega with his mother. When the bombs fell, they didn't make it to safety. He can be found 210 years later roaming the Commonwealth and Boston with Rosa and a few other pre-War citizens turned feral.
After the nukes fell, Codsworth tells the protagonist that he had seen "Ms. Rosa's boy running around in his Halloween costume," implying that Codsworth had seen him as a ghoul and mistaken his ghastly appearance as simply a costume.
In cut terminal entries, his name is revealed to be Louis, and that his father died, and that he and his mother building the Corvega together strengthened their relationship after the loss. Louis wanted a roll cage and a leather interior for the car.
A mailbox in the post-War Sanctuary Hills has the name Rosa printed on it. The house however, was cut from the game.
↑"Kind of crazy to think of how far Louis and I have come these past four months. And I'm not just talking about the car. I hate getting all poetic, but rebuilding this Corvega really has helped rebuild our relationship. He still misses his dad - God knows we both do - but he's learning that his madre ain't so bad. As for the Corvega: Most of the body work is done, generally speaking. Might not be pretty (yet!) but it's there. Interior needs new upholstery. Louis insist on leather - we'll see. - Louis wants a roll cage. I don't know how fast he wants to go, but I promise him we'll at least achieve the baseline 0-60 in .5 seconds. "