Very true! Imagine there's a bit of a vicious cycle there. He may have always had that sort of relationship with his son, which made his son less likely to see him as an authority figure, so if he suddenly morphed into one, it's a "Hey, why are you telling me what to do?!" situation.
And I imagine AS realized, even if he didn't want to admit it, that the relationship he and his son had, which seemed good for a couple of years, was still tenuous and built on a fraught history. So, he's not going to jeopardize it for appearing shrill and smothering while he's on a roadtrip with his buddy.
Interestingly, didn't BS keep up with grandma until the 14th or the 15th? Wonder if he accepted her as more of an authority figure and made a point of staying in touch with her longer because he knew she'd be more vocal if he didn't. He also certainly gave her an excuse for leaving that sounds like something you'd give someone someone you considered an authority figure. Rather than the vagueness he gave dad, he gave her a specific location and a specific reason.