I wasn't a child of divorce, but i was 11 and my sister was 9 when my brother was born. I am going to echo a lot of what the PP said.
There were a lot of fun things about having a much younger sibling, but I was pretty much relegated to the job of "assistant mom" as soon as he was born. I was the babysitter or mom's helper whenever they needed it: "help your mom and change your brother's diaper" "Get your brother to eat his vegetables" "Get your brother ready for school, I need to do xyz.." All those requests don't seem like much, but if they are happening every single day, it's a problem. Make sure your son still gets to be a kid.
For now I think the best you can do is let him know that while things will change for everyone when the baby gets here, he'll still be doing all his favorite activities. And he will still have some alone time with you.
A few things for after the baby arrives: If he doesn't want much to with an infant, don't pressure him, they're more fun once they are mobile and start interacting a bit. Let his room be his room, no pressure to babyproof, etc. And if he gets tired of a grabby toddler getting into all his stuff, respect that.
I think have the help of a therapist will be a great benefit for your family and your son. It sounds like you are headed in the right direction.