Tbf, he told her the truth in ch. 19 (a halfhearted one, but he was right when he said he hasn't money to buy prostitutes), but she's too drown in her relationships ptsd to actually listen to him. Even though he tells he's going steady with a good girl, she won't believe him. And remember what was her reaction after suggesting he might get a girlfriend back then in ch. 7... Yeah, putting him into child slave jobs.
So if you think MC is a dumbfuck as of now, when he finally finds out the truth about his mom's activities, you'll find him as a callous asshole bc his reaction won't be pretty to see. But nvm, the key point here is communication.