I had a 2 year old when my second was born these are some things we did to help with the transition.
We started talking about the baby right away, and when I got bigger, we encouraged her to kiss and hug my belly/her sister/the baby.
We put the swing, bouncer, crib out ASAP. Maddy got out of the crib in February and we immediately set it up in the nursery with the glider and changing table. We called it her sister's room.
We got her a doll for Christmas. We would pretent to change diapers and feed the baby. She would take naps in her crib, swing in the swing, etc.
In March we moved Maddy's carseat to the other side of the car, and in mid-april, we put the base in the car for the baby. Sometimes, we would pack up the doll in the carseat and take it to my parents house. We would get Madison involved in all of these activities and she loved it.
Once Brooklyn arrived, when Maddy came to the hospital. I made sure I did not have the baby and spent a few minutes playing with Maddy on my hospital bed. Made sure she got mommy time before meeting her little sister. Then, when we came home, we brought a present for Madison and told her it was from Brooklyn.
Once home, we made sure to have Madison help in anyway she could, bring diapers, wipes, burp cloth, etc. While I was nursing, I would often read to her, and while Brooklyn was napping, we always got mommy and maddy time. If the day was crazy, Madison adjusted well, and we would have our time before she went to bed.
About 3 months after her sister was born, we went out, just Maddy, Mommy, and Daddy, to Disney Playhouse Live, and I'm taking her to Disney Princesses on Ice tonight, with no little sister. Just make sure you make time to spend with just him alone, even if you can't go out with him at first. And when you can leave the baby, then take him to the park alone, or to et some ice cream, or just something special.
I hope ese ideas help.