Hmm, maybe. Yes, there's a lot of history structuring those choices, alright.
Rusty's relationship with Roy is strained, but is within rather broad bounds of normal (and Rusty wants to keep a line in to his brother as well). August has been read-in on this nonsense (a little, it seems), and Rusty's right about her capacity for emotional support (and smarts to recognize the situation). Bringing Maddox would have turned this into a visit-with-the-Kingsley's which wasn't the main point.
Rusty is also choosing not to expose Matty to toxic behavior. Rusty's mother didn't say anything really awful on that front, but Rusty might have intuited that bringing Matty would have been a coming-out moment and might have triggered a gusher of narcissitic venom. August is a shield from that, but not exposing Matty to that is the right thing to do. Sure, it's also a ruse, but his mother isn't entitled to know if she's going immediately weaponize it.