Yagami-kun no Katei no Jijou

Well, in short, the mother looks like she's 14, and the kid is "openly" in love with her—loli x motha-con. His teacher, too, is in love with his mother, and so the love rivals duke it out in classroom, as well as plotting the downfall of their common enemy, his father. The father has a secretary who is in love with him. But once she discovered that he has such a "young" wife she turned her attention to his son, who is now torn between his own mother and a naive, motorbike riding girlfriend.

(Source: users.skynet.be/mangaguide)