They were neglected, yeah; the lack of medical records proves that. The lack of school records, too, if they weren't educated somehow.
I don't know if that means they aren't autistic, though. Autistic kids are more likely to be targeted for abuse or neglect than their non-autistic siblings, so it's common enough to see both. In an abusive family, it's the autistic child who's usually the main target. But then, a few abusers actually label a non-autistic child "autistic" to try to justify the abuse. Yeah, I know it's messed up, but I've found multiple instances of it.
By "autistic" they might have meant "mentally impaired; unable to communicate". There are of course multiple reasons a child might be unable to communicate. Without medical records, we can't know until (hope, hope hope it's until) they find this boy and evaluate him.
If someone killed this boy, or if he died and they hid his death, I hope whoever did it comes forward and admits to it. Until they do, I think if they have any conscience they will be tortured by that secret, by the knowledge that this boy has siblings who need to know what happened to him. Even for a potential killer, confession and prison time would still be better than having to live with that, I think.
On the other hand, there's the very rosy picture of a friend or a family member who doesn't trust the police, taking the child away from the family, kidnapping him because they know he's in danger and they want to help him... It may be just wishful thinking, but how I hope it's true. Maybe Jason is out there with someone who loves him and is hiding him away, trying to figure out whether it's safe to come out, whether now that his parents are in jail, Jason might finally be safe. I know. It's far-fetched. But I don't like to give up hope.