I think i have a rough idea why hes crossdressing now, its so he isnt killed for potiential succession of the throne, from the painting it seems they have a legitimate child and if they were to find out shes a he than he would immediatley be killed for being the 2nd son of the emperor and having succession claim if the 1st prince were to fall out of line of succession, so probably his mother hid the fact that shes a he so he wouldnt be assassinated by either the 1st princes mother or the entire imperial family if the emperor also didnt want another heir