It was a very looooong time ago now (early 80s), and so probably not at all relevant, but I did this move when I was almost 7. We moved from Edinburgh to London and I started in what was then top infants (so year 2).
I was absolutely streets ahead. So far ahead that the school didn't really know what to do with me. In the end I skipped first-year juniors (year 3) altogether. Which then caused problems at the end of primary school because the LA wouldn't let me go to secondary a year early so I ended up repeating the year.
I went to an inner-London primary school where lots of kids didn't speak English and where it was very multicultural so I had no problems at all with my accent or anything like that. When I went to a 'naice' secondary grammar school, though, it was a different kettle of fish. I was tormented so relentlessly that in the end I adopted a tactic of speaking in an English accent at school, and Scottish at home.
Emotionally though it was fine. I was happy as long as I was with my parents and my sibling. I'm sure your little boy will be similar.
Also, my son is an April baby and his friend, who lives in Edinburgh and who's 3 weeks older, started school a whole year later than him. However as far as I can tell, they were both exactly at the same point when they started y1 and p1. Reception in England is very like nursery in Scotland.
Don't worry. I'm sure it will all work out, especially as you seem very committed to getting it right.