If a kid trumps your ace by moving out, fine. If he never sees you again over this, let him be your guest. Blackmail over these issues should not be allowed. Any kid to abandons and cuts off his parents over an issue like this is a bad bad kid.
I don’t mean he has to be mean to the kid. Just a nice, “no can do” works. No lectures, etc. Just no.
RG is entitled to whatever he wants. But, if the stepson decides to rebel, it has the potential to become an issue between RG and his wife, her assurances not withstanding.
It if gets to that point, it doesn't matter who is right or who is wrong: everyone loses. And the fallout can spread far beyond the original people involved.
(Been there, done that, got the T-shirt)
Shades of the prodigal son - IMHO, it will occur to him that he owes far more to his parents, than they do to him. Likewise, what they can give him far outstrips what he can give them. Just because they have graduated doesn’t mean they have nothing left to learn...