Quoting Mama MacGyver*:" I'd definitely work to fix it now, at a young age! My ds went through a similar phase, and DH would always ... [snip!] ... to my DH.. If he listens when your SO gets onto him, have your SO get onto him about not listening to you... Worked for us!"
We started kind of doing time out, when he is naughty we put him on the couch and tell him to stay, with DH he will stay till he is told he can get up (after a minute or 2) but I tried it and he listened the first time, after that he just ignored me again.