What about making a play on the names like "Mini Mouse" and being an actual strawberry shortcake (costume, not food, of course...hahaha), OR making him a prince with a Princess attached to him, made out of a sheet (face drawn on, hair attached), like he rescued her?
You may just want to talk with him about why he likes the "girl-thing" costumes. Is it their grandeur? Colors, as another poster suggested? Styles? I wouldn't push the questions on him, because then, you'll just make him feel embarrassed and ashamed for telling you, and you certainly wouldn't want that. His feelings and desires are important to you, and you want him to know that. Maybe you could go to Wal-Mart and look at the McCall's book or something and just talk about it then, very lightly, to figure out what his real interest is.
Another suggestion would be to go online and talk about what you'd both like in a costume. Maybe you could look for one, too, with him. This way, you're spending time together, working together, you're still acknowledging his feelings and wants, considering his feelings, and helping figure out what exactly is attracting him to the costumes without embarrassing or shaming him.
Just be sure to be sensitive. You don't know what his motivation is, but you don't want him to feel so terrible about his desires that he doesn't share things with you anymore. So, be loving, caring, gentle, and understanding of his feelings. I hope you all find something wonderful together!