In your situation one option I would suggest is to purchase (used) the DVDs and test booklets for Math-U-See as far back as the two of you think he needs to go. Then I would have my son watch a video, take a test, correct his mistakes. Watch a video, take a test, correct his mistakes. He should easily be able to get through two weeks of content a day, maybe a month or more in the lower levels where all he needs is a refresher. I’m not familiar with the curriculum you asked about- hopefully you will get a response from someone who is. Theoretically my suggestion should work for many different curricula. But I know from experience it works with MUS.
(You May even be able to borrow the discs and make your own tests by taping over answers in filled out workbooks if that’s all you can swing right now.)