I think you have had some good replies already and you’ve been defensive and I think that’s because you’ve probably realised you’ve been a bit naive about the whole thing. Please remember that most parents replying probably have more experience of 7 year olds than you do and therefore more ability to empathise with a child that age. That isn’t a criticism of you at all, just the reality. @KayPopin particular gives good advice based on her experience in a similar situation and you’d do well to take it on board.
I’m divorced and have a 10 and 7 year old. I think what you need to remember is that a 7 year old is still a young child. When you have your baby you’ll probably look at your stepson and think he is huge and mature in comparison.
Your post about the number of holidays and Christmas presents says quite a lot. My kids would much rather mummy and daddy lived together than have more holidays. It’s not going to happen. It can be hugely disruptive and unsettling going between homes and it’s our job as adults to mitigate it as much as possible.
What I would suggest is if you do get pregnant, once you have told your stepson, just make it part of everyday life. It’s obviously a huge thing but carry on with your life as before as much as you can for him. Ensure it doesn’t affect him and the schedule too much. Involve him with simple fun things like choosing clothes and a cuddly toy for the baby. But, don’t make every single weekend you have him about the baby. There needs to be a balance. Those trips to choose a pram and car seat etc are really boring for a 7 year old, so do those another time. A baby is a big deal for you but for him try and make it just another thing happening in your life rather than the central of his life. So, your DH needs to be doing all the normal things with him. Days out, 1:1 time with him as much as possible. Carry on as you are.
Once a baby is here don’t push the relationship. Again it’s easy to think a 7 year old is grown up compared to a tiny baby. Offer opportunities for him to “help” by choosing what the baby wears that day if he wants to but don’t push it. Accept that he is a young kid at school and will bring germs into the house and give your baby a cold. Do not be precious about germs or the odd bit of dirt. Don’t be cancelling the weekend with him if you’re all tired because baby has been up half the night.
I can’t emphasise enough that your schedule with him should not change or be disrupted as far as possible. Unless you’re in hospital giving birth your DH needs to keep to the schedule with him. Once you get home with the baby your DH needs to be focused on stepson whilst he’s with you, while you focus on the baby.
Don’t make him share a room with the baby or move rooms. Any changes that you make as a result of the potential new addition needs to have your stepson at the forefront of your mind.
You asked for advice on how to make sure he’s ok with this step, but there’s only so much you can control. He basically still needs to be treated as the young child he is.