What (if any) endorsement do I need to give a student who scored 100% on his knowledge test?
If we're talking about FAA regs I'm not aware of any endorsement required if your student passes the test with a perfect score (they need an endorsement to take the test - I have given the instruction required by..., and they need an endorsement like the one in your question if they fail or don't manage a perfect score).  

The endorsement above is intended for use when you need to endorse someone who failed to re-take the exam (similar to what's done if they fail a checkride), or when there's stuff to review from the knowledge test (My instructor combined it in my Checkride endorsement - And has demonstrated satisfactory knowledge of the areas found deficient on the knowledge test.

That said, it is probably a good idea to cover anything the student missed on the exam or doesn't feel 100% confident on, and to log that training - in this case if you do additional ground work you would just log it as you normally do...