Coming back to this because I feel that I gave a misleading impression by omitting a caveat / alternative course of action.
One, absolutely possible and positive, course of action would be simply to enjoy reading to your DS throughout the holiday. When he returns to school, arrange a meeting with his new class teacher to discuss your observations and concerns in detail and dispassionately.
My impression from your later posts is that your DS gave the 'appearance' of reading throughout the year when you read with him, remembering whole words given to him by you or by his teacher when reading books through several times. The teacher may also have observed this 'appearance' of reading, along with a separate assessment of his phonics through his daily phonics lessons, and may perhaps have been more positive about his reading than she should have been (or equally, know exactly where the gaps are, have a plan for working on them, and observe that he is making good progress towards reading - reading is, after all, not an 'all or nothing' skill, there are steps in the process - and therefore be happy with the progress he was making).
What your observation of his reading over the holiday (I presume that you were reading very similar books to those he was reading in school, or phonics books like Songbirds from the library - this is important, because obviously a change to books which have similar numbers orf words but not a controlled vocabulary [many general picture books are like this] or non-phonic books if he's used to phonics could throw him) adds is that his knowledge of reading is not secure, as he appears to have lost significant ground after a break, and that his ability to decode unknown words using phonics is not secure.
These are new observations that a teacher would be interested in - after all, in school there is not usually a break after which what has retained can be observed. If you could add to these observations in more detail e.g. he struggles more with longer words, or words which contain sounds written with more than one letter (e.g. th) or words which do not obey 'the rules' he knows so far, then that would be even more helpful to the teacher - but equally, you can simply say that you did not investigate any further and ask him / her to look into such things instead, as that would be a normal part of their job.
What I outlined in my earlier post about phonics is another possible and positive course of action, as you seem an involved and concerned parent and might want to 'do something' as well as wait for next term. Of course you do not have to teach your child phoncs - but if there is a particular phonic sound he struggles with, then a few days of spotting that sound in every possible context in your environment may well help in securing it in his memory, and it's a lot more positive and fun than both of you getting upset about books he can't read.