Baby dolls aren't just for girls, not in our house anyway. Pretend play might be a good way to help Matthew prepare for a baby sister.
The baby doll stays in the baby room. I only allow Matthew to touch her with my supervision, just as I would with a real baby. This also means no poking baby in the eye, no poking anywhere on the face and no pulling on arms or legs.
When I first started this "game" with Matthew, he wanted to climb into my lap and snuggle with me too while I was holding the doll. An early sign of jealousy? Maybe. He has only done that once though.
So far he's very interested in the doll and gets excited when I say "It's time to feed baby." I do this at random times throughout the day. He signs "baby" then heads for her room right away and watches me sit in the rocker to give baby a bottle. Sometimes we tickle her tummy (I squeeze it) and she giggles. Squeezing one hand makes her cry. Squeezing a foot makes her babble. Of course, he doesn't know that I'm squeezing her. When it's time to put her down for a nap, he willingly and gently kisses her head.
I'm hoping that this affection toward the baby doll will transfer to his baby sister.