He's a psychotic.
Hospitalized at age 7 because he threatened to kill his mom.
Hospitalized again at 16 for threatening to kill his mom with a knife
Poisoned the family at least 2 times with epson salts and claimed 'well dad did it to me because I thought it was a laxative."
in 2016 his brother joe had a car accident and Andy might have cut his breaks causing it.
Poisoned his dad with sleeping pills and antifreeze, nearly killing him.
Andy is smart enough to understand and navigate police and medical procedures to the letter, but his goals are entirely in the imminent present. He wants things now, like a child, and he doesn't want the good times to end. Its why he "runs away" or threatens suicide when someone doesn't give him what he wants.
We're not fully convinced he's smart (definitely smart enough to navigate the american health care system,) but he's definitely not the dead-brained vegetable he claims to be.