What are some ways to prepare my four year old autistic step son for a baby sibling?

My oldest son has Aspergers..and this is what I did with him with both of his siblings:
He came with me to all of my appointments and the doctor let him help with the finding the baby's heart beat and measuring my belly. We read 'sibling' books every night..When the baby starts kicking and wiggling..let him feel, encourage him to talk to your unborn child..make him a huge part of the process..picking baby stuff, to decorating..The hospital that we went to had a sibling class a couple of weeks before the babies were born and he was able to go see where I was having the baby, learn a little bit about the process..(depending on the level of autism you might want to ask if you can go several times to make sure he understands fully) and have your husband and yourself carve out something special that is just for him..a place, something you do with just him..so that when the baby comes he feels like he still has something to hold on to and look forward to..and be consistant..