His body never returns to how it was before he got pregnant, but he doesn't care that much about how he looks anymore. He cares more about his family. After all, his belly is a souvenir of the pregnancy that made him who he is today, and the baby loves sleeping on Daddy's tummy!
Imagine your favorite character is mpreg'd, and his big, round belly becomes a symbol for how he's grown as a person. As the baby grows, so does his heart, and after he gives birth, his postpartum belly eventually turns into a soft, warm potbelly.