we got a baby doll for our son just before his sister was born. he was about 1 at the time. Partly so he could feel important feeding his baby while I feed baby sister. Even now, he's not 4 he'll still sit down and rock & feed Lillie's dollies. Things I've read is that this kind of play helps boys explore a not so rough n tumble side. and can also help them learn compassion because they are "caring" for the baby.
As long as he's not obsessing over it or still carrying it when he's 12, then don't worry about it.