According to his mom, so take that with a grain of salt, Lucas was a normal kid until he was about 2 or 3 and had an "unexplained" high fever and fully body rash for like 4 days straight and after that he was "different".
So, if that's true, that is a high enough temperature, especially for a toddler, to cause brain damage if it really lasted that long.
More likely is that Lucas was always autistic it's just that, at least back in the early 80s, most parents had no fucking clue what that was because all most people thought of was someone who had to be institutionalized and couldn't communicate (or, at best, something like Rainman), and all they noticed was by about 2 or 3 their kid was 'suddenly' weird, like having meltdowns beyond normal toddler shit, not being able to play or interact with other kids well or at all, problems with eye contact, inappropriate behavior (again beyond what's just a normal toddler exploring the world), but as that was long before the "autism warrior mommy" culture bullshit and most of the time the only people who got that diagnosis were people who needed institutional level care and could barely, if at all, communicate with other people, kids like Lucas just got shoved into the, "Well, he's just a weird kid" category.
Couple that with an enabling, coddling, helicopter style parent that Myrna seems to have been, and you get what could have been someone that might have been reasonably functional on their own turned into...Lucas. She never bothered to try and teach him skills to cope and interact with the world around him, she just told him he was a good, sweet, perfect boy and nothing was ever his fault it was always someone else's fault if he got excluded, hurt, ignored, attacked, criticized, etc...
Whatever the reason, I'd be surprised if Lucas has any cognitive function level beyond age 10-12 or, if I'm being generously kind, 15 or 16. He doesn't seem to have mentally matured beyond being a teenager, at any rate.