Took me a "few" But I finally got the Gin scene, Boy wanted to grow up but he kept the Dress.
Gonna be a weird way to grow but grow nonetheless.
After A Maid session with Gin, wait around till only Gin disappears, Only Gin, if Cassie is gone as well it won't work, Catch him Cleaning his Panties while wearing a maid Dress first, then you can Wait till Cassie goes Fishing with Gin at the lake at the same time.
Then and only then will he go.
How do I get him to wear a Dress?
Spam train him, you'll get a new Option eventually. Make sure you have a full set of Maids Clothes, Headband Etc...