Not his father

[Clodfobble]

You need to have a conversation with the mom about the discipline plan you are going to enforce together. As long as he knows he can go to her and she will give in, he will never respect the authority of either of you. In fact, as the (sort of) step parent, it's actually preferable for her to do most of the disciplining at first, while he gets used to the idea of you as a second authority figure, but that means she's going to have to step up and not "melt" when he demands things. Work out a plan together--and genuinely listen if she thinks something honestly isn't worth enforcing, come up with a set of rules you can both live with--and let her know you will support her in her efforts to get her son's behavior on track.