Re: If you are changing curriculum now...
It would depend on his age.
For ds who will be 6 in April, we are switching curriculums. We totally abandoned our books and moved on. We started with about half of the material for his new curriculum, but for the K level.
If you are starting him with the new curriculum in the same grade level he was currently working on, then I would probably just take a short break and start on it right away.