Mamas with much older children and a newborn

I’m 20 weeks pregnant with my second child. My son will be 11 years old the month after she’s due. This pregnancy came as a complete surprise as I never planned on having anymore kids. My son is a complete mamas boy (his father hasn’t been around since he was 1). He is obviously spoiled, but when I say spoiled I don’t mean he gets any and everything he wants. He is spoiled by my undivided attention, my love and affection. Of course I’m biased but he is the greatest kid anyone could ever ask for. He is so loving and caring and would do anything to make someone smile. He has been my rock through all of our struggles. I honestly believe that I need him more than he needs me. When I announced to him that I was pregnant, he took it extremely hard. I expected his reaction to be as negative as it was and I completely understood why too. It’s been a few months and he hasn’t exactly came around yet. I’m not expecting him to be 100% okay with this just yet, but I would like for him to be just a little more happy about being a big brother. I have taken him to two sonogram appts with me hoping it would start to open him up. I know he will be an amazing and protective big brother, but I’m also really nervous that once the baby comes he will start to act out and not be the same sweet boy that’s all about his mama anymore. I know with his age, it’s bound to happen anyways where he starts to pull away and try to be his own independent person, but I don’t want him to start acting out bc he feels replaced. He is so sensitive and takes everything to heart. Also, with his father being out of his life, I really don’t want him hurting bc his little sister gets to grow up with her dad....we have a few months until she’s here, so I was wondering if anyone has any stories or advice on how to help prepare him and myself for this new adventure and help open him up to being a big brother? And any advice on how to basically start over with a newborn lol.