okay, let me clarify the using of the doll (and the using of the doll example comes from naomi aldort and "raising our children, raising ourselves" and she is one of the most gentle people on earth!). YOU may have problems with the boy "beating" up on a doll. YOU want him to beat up on a pillow because it makes YOU more comfortable. YOU are able to think/say/believe that the pillow represents his little sister. But what if HE can't relate that the pillow is supposed to be used so that he doesn't hit his sister. if he can use the pillow, that is fine. But if he needs a doll, than that should be fine too. this is not about you, it is about HIS needs. he may need a doll to hit because he, unlike YOU, can't abstractly think that the pillow should be like his sister.
and this subject is very near and dear to my heart. i had an older brother who beat me up daily until he finally killed himself at 19. he broke my nose, ribs, etc. My parents took away his toys, locked him in his room, etc. but they never TALKED to him. they never acknowledged that is just plain old sucks to get a new sibling, especially a sibling that might be smarter, cuter, get more attention. they never approved of the way he needed to physically let go and so he never learned.
so remember, it is you who are afraid of the doll because you attached a story to "what if he hits a doll." the story you attach to hitting of the doll is that he won't be able to differentiate between the doll and his sister. or it is against our culture. or it just sounds bad. or what if someone finds out. but that is YOUR story. it is not HIS story. let go of the story and follow his lead, his needs. and open your arms to reality.