To make a decision I think you still need to know what support the other school can give him to help him catch up and whether this will make enough of a difference. The other thing is that occurs to me is that if his new school is 30 yr 3 children it is possible that in a split yr2/yr3 class there may be more children academically closer to where he is than in the all yr 3 class (iyswim)?
To catch up he needs to be progressing at a faster rate than the children learning around him. IMO the best way to accelerate progress in relation to peers is to do extra work out of school. It's a tough call because this can be a challenge to accept/fit in, but if he is wiling and you are able to support him this is likely to pay dividends.
I also speak from experience and have been amazed at how just half an hour of maths most days (but by no means all) has help DD move from the support group into the top set. Not to mention how supporting DD with her phonics has helped my own atrocious (i.e. mildly dyslexic) spelling.
My approach was to pick some workbooks that were at the level below where she was to build her confidence (which was very low) and it worked. Doing easy work helped boost her confidence so much and ensured it was easy and as fun as a workbook can be (& she called it fun maths as she preferred it to work at school which at that point she was struggling with). The regular practise of the basics helped secure some essential things & made the gradual progress forwards possible. Of course there are other ways to achieve the same goal, tutoring and on-line programmes for example, plus a ton of help right here on mumsnet!!!
If you really want him to make accelerated progress, which is what catching up is going to mean, then I recommend you consider which approach will work for your DS. Maybe the move to the new school will provide enough 1:1 support to enable this accelerated progress, but you need to be fairly sure, otherwise whilst he will progress, he is unlikely to catch up.