Switching from BJU Math to CM Math, 2nd grade boy

My son is currently in a homeschool co-op that uses BJU curriculum for Math and the XtraMath.com app for the development of immediate recall of math facts.  He does well in math and grasps most concepts easily.  However, he has had significant trouble learning his beginning subtraction facts (0-10 families).  We drill flash cards, worksheets, spent a good bit of time using math cubes, online games to reinforce but he is still struggling with about 15% of those facts.  He has to pause and calculate.  He followed the second grade material all through this year and learned addition and subtraction facts through 20, as well as place value, comparing values, money, renaming, carrying over, etc. and he did well with all of those things.  But I didn’t make him memorize any of the new math facts because he hadn’t completely gotten the beginning facts down.  I don’t want to move him forward in BJU math because the third grade material begins with multiplication.  Technically, according to BJU standards, he is about a year behind in his math.

We won’t be involved in the co-op next year and I’m switching fully over to CM.  But I’m curious what do in regards to his math studies.  I looked at the CM Book 1 kit and it looked a bit basic for him.  I had trouble finding an equal amount of info on the CM Book 2 kit.  I read over some forum discussions and found where older students who had switched from formal curriculum to CM were encouraged to start over in Book 1.  I was a little unsure about this.  I also have a K5 student who will be starting with that kit this year so I imagine it wouldn’t be hard to teach them together but I suspect that my son will move ahead quickly.  Ideas, anyone??