Sure! I wouldn't sweat it. Some boys carry around baby dolls. Some girls play with trucks! He could like dressing the doll because he is copying you getting dressed. Nothing wrong, he's just a little guy.
yes it is, my brother is 6, and i'm 13, as he was growing up he was asking for dolls, wearing my clothes, wasting my make-up, playing with bratz dolls and everything, sometimes now he still plays with them with my 2 year old cousin, but its nothing to worry about, its just how they want to be, my brother is now into boxing, wrestling, karate and eventually your brother will grow out of stuff like that, if he doesn't, well.. you shouldn't be bothered in how he turns out, hes your brother and you should love him for who he is :)