My daughter is in Year 1 at NP and we love it. I'd say that it is academically ambitious, but I certainly didn't feel that in Reception which was a good mix of fun and learning. More so this year which is definitely a step up, but the kids seem well supported through it. They try and get them a year ahead of where they should be so there is time to 'prep' for senior schools come that time but I imagine this is the case for most independents these days? It's buzzy - there's lots of extra curricular stuff and some amazing facilities too. We also didn't go to the nursery but that was the same for around 50% of the kids in our class so your DS won't be the only one. Good luck for September, I'm sure you will be reassured as soon as you start.
Edit: Also noticed you asked about NP being nurturing - I'm not an expert on either academics or the softer side of schooling but for me, there does seem to be a lot of focus on how to be a good friend, wellbeing and staying healthy and happy. My daughter has loved both of her teachers and her TAs which I have taken as a good sign. I've found them open and honest and willing to talk any time. I hope that helps but if you'd like to chat at all, let me know