How about this, exploring mom's motivations a bit:
Mom is a high-powered executive used to bossing people around.
When her son approaches her to tell her about his new hobby, she thinks it sounds ridiculous. And besides, shouldn't he be spending his time studying? He's doing terrible... But what the hell, she's been wanting to stop smoking for some time, and who knows, maybe that's the thing that'll work?
He goes into the whole spiel about following his voice down a staircase while counting backwards. She decides to humor him, all the while smirking inside, thinking "does anyone really believe in this crap?"
"Alright mom, nod if you're completely under." She nods, trying hard not to crack a smile or start laughing.
He's thinking, wow I can't believe how easily she went under... "Okay, everything I say will sound reasonable to you, you can trust me in all matters..." Where is he going with this? thinks mom. How can he possibly think it will work?
"Cigarettes are nasty and a waste of money," he goes on. Yeah I know this, she thinks. Everybody knows this. I should play a trick on him, get him all confused.
"When I snap my fingers, you will remember everything I said subconsciously, and follow my instructions." Right, she thinks, I'll get right on that. I do think I'll quit on my own though, he needs some encouragement. It'd be a nice fun success story to tell later on.
So he snaps his fingers and mom makes a show of blinking her eyes as if just waking up, looking confused. "So when do we start? Are you gonna hypnotize me or not?" she says, trying her hardest not to let on that hypnosis isn't real.
"I just did," he says, looking smug as hell, feeling like the king of the world.
Mom stops buying cigarettes of her own volition, finally having a good excuse to drop the habit. She just up and stops, throwing the half pack in her purse away and not looking back.
Some days later, she tells him to clean his room. He jokes, "why don't you do it?" like so many times before. She pauses for a bit before yelling at him. What if I really do it, that'll really burn out his brain. And so she starts cleaning his room. The son has no idea what is going on, trying hard to think. "Eh I was joking mom?" She decides to play the long con, "well it sounded reasonable coming from you... I trust you in these matters."
Holy fuck, he thinks, those are my own damn words! I must be the greatest hypnotist in the world! "Uh yeah mom, of course."
The next day, he comes home with his report card. His grades are terrible, as expected. They get in a horrible fight, yelling at each other. Finally, she says "and you spend all your time on hypnotism instead of doing your homework!"
Infuriated, he yells back, "oh blow me mom! You stopped smoking didn't you?" She stops in her tracks, realizing what he said. Did he mean that? She sees him in a whole new light, has he been having thoughts about her? Is that why he hypnotized her?
Does he want me to blow him? Do I want to blow him? Do I want to do what I'm told? I always tell others what to do... I can use this to my advantage. I can get what I want, without blame, without guilt.
She approaches him, still flushed from the anger she just felt, but starting to get aroused as she gets on her knees in front of him.
Who is right? Who knows? Who cares? Whatever the cause, the outcome is the same