I am from a giant mixed family, and there is a 34-year gap between my oldest and youngest sibling. There is a 19-year difference between myself and my youngest sibling. There are 8 of us in total, for context's sake.
My oldest sister (my older brother is not with us) was absolutely horrible the entire process. She was 31 when my younger sister was born and you'd think she was a child. My older sister had full on meltdowns about how it wasn't fair that another kid was coming into the world, and how my sister would have a much better like than all the rest of us.
I was kind of surprised and worried at first, but once I met the baby I was fine. I was never overly angry, more just confused at my own circumstance.
My sibling closest in age (a year younger than me) resented my youngest sister for a very long time. Her old room became the babies new room and at the age of 17 my sister would scream and yell that she had been replaced and that no one cared about her.
Everyone else was either happy with the pregnancy or didn't say anything.
My oldest sister is still a shit about it, but she is also a shitty person. All of the rest of us are involved with her and love her.
I think the big thing is breaking down the demystification that is babies. Some of my unease came from the fact I had no real experience with babies and was scared of their fragility. I remember coming home from college break and I would hold her and give her a bottle and it was a great bonding time. It was the little things that connected us, like the nickname she would call me, or being able to play with her and make her laugh. My step-mom would constantly offer these opportunities to me and while sometimes I wouldn't take them, the majority of the time I would.
I know you say you've assured him that when the baby gets here your son's life and activities will continue, but I would go out of your way to explain the plan and how you're ensuring it will maintain. Show him you have a person who can drive him, someone, who can come to his games, or a babysitter or family member who can watch the baby so you can spend time with him.
When the baby gets here, he is going to see how tired and stressed you are. Rather than trying to hide it from him, recognize it and give it a name. Let him know that yes, you're tired, but just like when he was a baby it is all worth it. Maybe talk to him about what he was like as a baby and engage him in conversation about himself that can get him more comfortable with baby talk. Find some pictures of him, a baby book, clothes- whatever.