Musumama 2

The protagonist, Ren, is an honor student, who is strictly educated by his mother and trying to succeed his father, who is a doctor.
He respects his mother a lot, but sometimes, he just wants her to be a little bit more lenient.

One day, he accidently sees his mother masturbating.
After that, when studying with her, he keeps thinking of it and gets excited.
Even though he is ready to be scolded, she calmly chided:
"So this is why you can't concentrate."
And promptly help him get off.

From then on, in other to help him focus, she decides to help him relief every study session...