It could be that the mom has a creative streak. She views dresses as a form of art and wants 'him' to be part of her creative exploration. Maybe she's a fashion enthusiast and sees no harm in having 'him' try on dresses to experiment with different looks and aesthetics, and also to expand his understanding of the world of fashion.
Perhaps the mom is trying to teach him about gender equality. By putting him in dresses, she's showing that clothing doesn't have a gender and that he should be open - minded about different ways of dressing. She might be influenced by modern ideas of breaking down gender barriers in all aspects of life including fashion.