That he has Xanax in his system wouldn't be a problem. Problem is that he overmedicates to say the least, and uses alcohol and other drugs with the Xanax.
I'd bet that if you're pulled over for driving like shit, speeding, passing on the shoulder, brake checking, etc., and you're slurring and acting out of it you're gonna get popped.
And if you show up for a visitation where you have to pass drug tests and you're slurring and out of it you're probably not gonna see your kid.
Having a Xanax prescription isn't gonna help much if you're slurring, out of it, and smell like a brewery.