he will have to get used to a whole new environment come 9 months later that's my only concern
I can see where you're coming from and I agree it would be better for him to be in a setting with close links to the school he will attend.
In a good early years setting, he will have the chance to get used to the routines of being cared for in a group. It is significantly different from being at home with someone who knows you like the back of their hand.
There are systems like maximum numbers at the sand table, listening for your name in a list, finding your coat in a scrum or having to go and get your own snack that he will need to get used to. In a nursery or pre-school there are more staff around to spot a struggling child, think through why they aren't managing and talk them through what to do. In a reception classroom it's a bit more trial and error.
The academic side of it doesn't matter at this age. What matters is that her feels safe and confident in the reception classroom because he is used to being cared for in that way.
ONly you can decide whether the advantages outweigh the disadvantages but, having spent a lot of time helping out in classrooms and having worked for many years in nurseries, preschools and as a childminder, plus 17 years of parenting, I would say that being equipped to cope in reception is worth the disruption of having to settle in twice in a short period.
If he doesn't get that experience, he won't be penalised and the staff will do their best but the numbers will make it harder for them.