How to make my parents step up to their parent plate?

Sounds like your parents may need some outside support. Child services cares about neglect as well as abuse. The state will have an additional interest soon, when he becomes old enough for mandatory public education.

If you decide taht it’s up to you to “take the lead and raise him,” what would you do toward that end?

he is still a kid. i agree with your parents and you need to understand it

As a person who filled the shoes of parents…It is A LOT of work. My brother didn’t speak to anyone until he was 4 and if you spoke to him now you would never tell but that is because I got him the help he needed. In all honesty your brother is not considered a “vegetable” that is harsh to say he is more of a under-motivated child. Don’t take the role of parent, instead spend time with him on Saturdays and Sundays. When our mother was here she was always busy and on weekends I just took my brothers to the park, and they really looked forward to it because it showed them someone cared. My brother even started talking more so (it was not words but close enough). And in all seriousness you should be looked down upon too for letting such actions go on for 5 years before doing something about it. Natural instincts should of told you to care for a human you love and care about.