regressed to stop listening.

I'm a nanny of a kid with teacher parents. Don't get me wrong, they're super duper amazing wonderful people, and lovely adoring parents. They're just so incredibly chill that it seems (once again) after the summer this little one has forgotten how to listen. I know that I'm stricter, I mean what I say and I was adamant on teaching him that no means no and that you do what you're told. Hes super young (almost 3 now), but I've nannied him for most his life and we were doing really well. His parents liked how things were and were appreciative, but because they're so relaxed I think the little guy got used to not having to listen/have much follow through, and I see it in his eyes that he's challenging me and knows what I'm talking about. It's only been 2 months. This happened last year as well, after summer he was much more obstinant and it took a couple of weeks to get things back to where he would listen.