I'm no legal expert, but I'm positive that the only situations where someone can be legally declared incompetent is if they're a literal pants-shitting retard or are so far gone mentally to the point they start harming themselves/others because the aliens in the walls told them to.
So unless the crack ends up completely frying his brain or some especially insidious rat convinces him that he'll win the Juice god's favor if he kidnaps and sacrifices a kid Austin's parents really don't have any options legal-wise to deal with him.