Walk in on him masturbating

"...walked in on him while he was masturbating. You could then tell him the story about him doing the same to you."

Yvonne looked understandably apprehensive. "That seems a little invasive."

"It is, but it would give you an excuse for why you need to talk to him now. Besides, you don't have to stare, just open the door and look away. I assume you already know when he's doing it."

Yvonne blushed. She had been carefully ignorant about what her children did sometimes, and while she had some assumptions, had often told herself that for all she knew, Toby could be doing anything.

It took a little longer to convince her, but the plan to make her less embarrassed by making her son more embarrassed was eventually accepted.

It was a few days later, and she had worked up the nerve to walk in on him.