Yes i believe he does because whenever minnie embraces atau flirts with him he seems annoyed. But whenever he gets close to mimi he blushes, hes also very sweet to her for example he tried to understand her when she was angry and vowed to protect her from any danger including his mother and her father, and when she was fighting the cerberus in his kastil, castle Jr. seemd quite impressed with her battle skills. So yes I belive he has developed some sort of crush on Mimi.
i think he does becoase he always blushes when Mimi holds him closely. if anda look at the previews for grim tales 'Afterbirth' its hinted they may have a relationship 1 link 2 link
Also his 1st crush was the ghost girl from "The Ring" she is "the girl from the well" minnie(in angry mandy mode) was refering to in the issue [Girl From The Well] link
It doesn't really say but he blushes sometimes when she moves close to his face and when he saw her underwear. And he also thought she was hot so it's not sure but i think he does
I checked his bio and relationships and yes he in fact does have feelings for her.In fact, It's because of his growing feelings that he pprotected her from his mom (and survived).