His friends like May and Max would be really surprised. May might giggle a bit at first but then feel sorry for Ash. She could try to find some cute Pokemon - themed clothes for him. Max would be curious about how this happened and would want to help find a way to reverse it. Professor Oak would be very concerned and would immediately start researching what could have caused this strange transformation, while also trying to make sure Ash is comfortable.
Pikachu would probably be very protective. It might zap anyone who tries to make fun of Ash. Misty would likely be shocked at first, but then she'd start taking care of him like an older sister, making sure he's clean and fed. Brock, being the nurturing type, would probably be the one to find practical solutions for taking care of baby - Ash, like finding proper diapers in the Pokemon world.