Guy turns into a baby

A mysterious scientist presents a timid chemist with an elixir which turns him from a weakling into a confident, vital male, able to defeat his rival and impress the girl of his choice. The only problem is that this mysterious substance has the unexpected result of causing him to revert gradually to babyhood...

A young father starts to feel increasingly displaced by his newborn baby.

Comedian Sal Howard has a big problem: his wife, scientist Dr. Dana Howard, doesn’t want kids. To save his marriage, Sal tricks Dana into thinking he’s become a real baby to show Dana the joys of motherhood. Sal’s harebrained scheme works. But the plan backfires when Dana develops a hormonal serum that can transform Sal into a full-time, adult baby– forever! Manbaby is an absurd, comic fairy tale about what it means to grow up.

A man has a baby, runs away, then comes back, but he can't just show up, apologize, and hope that makes himself a grown man.