I think at the beginning Tsukasa doesn't want any help to extract his breastmilk; his body feels weird, and his nipples feel weird, and his chest feels weird. The sensation of the breast pump is already weird enough. But oh is Rui good at it, way better than the machine he got to pump the milk out.

Rui is horny yeah, but he is also so incredible gentle. His large hands cup Tsukasa's tiny and swollen tits perfectly, and his mouth feels so warm against his areolas. Tsukasa tries his best not to moan when Rui's tongue lick his sensitive nipples, but soon he is crying as Rui's gently sucks the milk out. The omega blushes so much noticing Rui's eyes looking at him.
The nights when his chest bothers him too much, the alpha is quick to notice. He lets his omega rest on his chest, and he gently massages over Tsukasa's soft post-pregnancy belly, holding the breast pump while using his fingers to help speed the process. He just wants the mother of his pups to be comfortable really.

I don't think he would have a mommy kink, I think Rui has a serious Tsukasa kink, in which everything Tsukasa does is way too erotic. I have one too. Shameful really. But it's impossible not to be attracted to him! Specially when he is cute and preggy!!