Paul Danos character was supposedly not able to communicate better due to years of being drugged.
How does that work exactly? At the beginning of the movie, he is telling Jake Gyllenhaals character not to touch him and answering questions.
So how come he wasnt of more help later? What drugs would make you become a mumbling idiot for days?
Laura:You left a dead prostitute buried alone in the desert?
Kyle:She's not alone.
I can suggest a number of reasons, obviously I don't know for sure as this was never fully explored in the movie.
1) Alex was never allowed to develop in a normal way. Lack of social contact especially early, likely abuse. This would also explain his reluctance to be touched.
2) The drug they were using likely had some side effects - be they directly damaging to the brain or like most sedatives leading to respiratory depression and possible hypoxic (low oxygen levels) brain injury (it's unlikely they were very careful). Without careful monitoring sedative drugs can be very dangerous.
3) Direct head trauma either intentional or accidental