As a college chemistry professor, I say do ALL the practice problems in the textbook. You could probably buy a different publishers old edition of a HS chem text for under $15. Every text is the same, they have different pictures, the material is the same. That would give you new practice problems. Beware of quizlet, it is student made, so it can have wrong answers. Brown and Lemay's Chemistry: A Central Science is one of the most popular texts out there, you can get older versions and maybe be able to find some teacher resources online.
I agree with his teacher, it is all about the practice. Understanding the broad concepts is not enough, he needs to be able to do all the little problems. The only way to do that is LOTS of problems.
I am very surprised that you are not finding it on Khan, that has the complete standard HS chemistry curriculum on it. The acid/ base section on Khan is pretty comprehensive.
What text is he using, what concepts are you looking for, you might just be searching in the wrong section.
For YouTube videos, you can try Professor Dave Explains and CrashCourse Chemistry.
Tracey
DD1 3/07 Itching to take over the universe.
DD2 1/14 My mellow little snuggler.