My son doesn’t seem happy at all about my pregnancy and I’m worried he won’t want anything to do with his new sibling

I (30f) have an 8 year old son who will be turning 9 right around the time I am due to deliver his new baby sister. He seemed less than enthused when I told him about the pregnancy. He knew that his step dad and I were trying to conceive because we had an in depth conversation about how we loved him but we also wanted a baby together (this is our first). When I told him he didn’t believe me. He said he knew we wanted a baby but he didn’t think I would actually be able to get pregnant (it took several months). Now that he can see I am indeed pregnant and the reality has set in I asked him again how he feels about it. He says he feels neutral about it and quickly changes the subject. I know it can be tough to adjust to having a sibling after being an only child for so long. He is very independent and I’m just concerned that once she is born he will further alienate himself from me in an attempt to not be around his baby sister. Has anyone dealt with this before and what did you do to help your child become more accepting of their new sibling?