Another English teacher here. I agree with the previous poster who said PEE can be very restrictive, especially with more able students. That said, it's sometimes useful for younger, less able students or those who simply prefer a set structure which is why some schools still use it.
We use it in lower school, though tend to use PETER instead: point, evidence, technique used and effect on reader.
Basically, what your son needs to do is make a POINT related to the question he's answering; give carefully chosen EVIDENCE (a quote) to back up this point; EXPLAIN who his evidence works (how it proves the point made initially and answers the question) trying to focus on a specific word within the evidence (quote) and the impact that word has on the reader.
It's important not to repeat himself and not to generalise (so avoid statements like 'this has a big effect on the reader').
Compare these three answers to see what I mean:
PUPIL 1 = He thinks they are scary.
PUPIL 2 = The writer thinks the Martians are scary: ‘Strange creatures’.
PUPIL 3 = The writer thinks the Martians are frightening: ‘Strange, swift creatures’. The first adjective ‘strange’ shows they are different and would have shocked the narrator at first.He also notices how fast they move: ‘swift’. The second adjective emphasises its speed and the fact it could easily overpower the humans. This is alliterated to add an increased sense of danger. The narrator would have found the creatures incredibly threatening.