We moved at the start of this school year, ds1 moving into year 4, ds2 into year 2. Wasn't worried about ds2 but was hugely worried about ds1 as he loved his old school and had a lovely group of friends. He's also really shy and sensitive.
We found it really helped that we were able to move at the start of the school year (meant driving an hour to school for a month until the new house completed).
Ds1 has LOVED it from day 1 and we've never looked back. He went to the transition day in the July and met a couple of friends so he had familiar faces on his first day, and now has a lovely little social group, I would say closer friends than at his old school. He adores his teacher too.
I would see if your DS can go and spend a morning in his new class (if travel distance allows), his old school can give him time off for this. I also found out things his new school offered like an allotment, forest school sessions etc, and used them as a selling point when talking about it. Same with after school clubs.
I would fully expect your DS to be sad about leaving his old school, and nervous about the new school but that passed in no time for us.
The only thing that's been difficult is me getting to know the other parents, which has then proved tricky when DS wants to invite friends for tea etc. I've joined the PTA though and volunteer for lots of stuff so slowly getting there. Otherwise all very positive and I'm glad we did it.