Looks like he's gonna share with his friend, Takeshi. This might actually turn out pretty wholesome, since the only one getting cucked is her already cheating husband, and she loves them both as her sons, and vice versa. Next one seems to have Takeshi as the protagonist, and Itsuki and him get along really well, so I do think I'm up for some brotherly bonding between Takeshi and Itsuki with their masochistic mom~
Link to the next and final chapter: https://www.dmm.co.jp/dc/doujin/-/detail/=/cid=d_304231/ 肝っ玉かーちゃん3〜大好きな母親とドスケベ種付け性活〜 The title reads "Very perverted impregnation sex life with my beloved mother", so maybe they'll take turns? Itsuki on that one night per week, and Takeshi every other day? Deffo going to have a threesome at some point, I'm quite sure, as they continue to train her.
Edit: It's a speculation from the title below! I am not in contact with the author in any way, so no more DM-ing me about "how I know" please ( ><)人 It might even be that next chapter, Itsuki's trying to show his Takeshi that his mom belongs to only him (Itsuki), but then Takeshi steals her back since he sees her every day through impregnation sex, while Itsuki only has 1 day a week with her! ALL SPECULATION.