Happy to try to help! That’s definitely a trickier proposition when you’re in Unit 3 where that background knowledge is so foundational. In my experience, a lot depends on where you’re at in the unit (like you said, the later you are in the unit, the more challenging it is to play ‘catch up’) along with the student and his/her abilities.
If you’re not too far along and can have the student complete the earlier lessons mostly independently (would obviously depend on the student), you could create a new Code.org section for that student and release those lessons as they complete them independently, checking their progress as they go. I use my own instructional slide decks in class that I’ve given to students to use independently, but you could also give them access to the Code.org instructional slide deck to use in conjunction to them going through the lesson if it helps. If the student can move fairly quickly through them, you could encourage them to do some work outside of class in order to catch them up.
If that’s just not possible based on where you’re already at or based on the student and you need a totally different plan for that student, some ideas that might work:
-Express Course (totally self-paced and would still help to build some of those Unit 3 skills)
-Code Break (from Spring of 2020 when the pandemic started…weekly lessons with the founder of Code.org and special guests exploring topics of CS and accompanying lessons)
-Have the student work closely with another student (if safe to do so) whereby the student can be supported…almost in a paired-programming type of setup. You’d obviously have to have the right kind of student to lead who is strong enough to be a quasi-helper/instructor for the new student.
It’s definitely not an optimal situation, but you can only do so much as one person leading a full classroom and trying to divide your time to support all learners. I hope this helps in some way…others may chime in with other ideas as well.