Personally if he is interested in reading the English books at home I would let him read them and encourage him. My son nay ever read in English at home and French at school for a very long time. He still prefers reading in English but he has done all his schooling in French and is in lycée in Neuchâtel now.
The OP clarified and said he was in first class which I guess equates to 3P in Vaud and 3H in the HarMos system so that would make him 6-7 years old I guess.
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Some children are quickly bored at the over-simplified, silly narratives in starter-readers.
I hear you that he cannot (or will not) read, not even single words. Even so, I'd suggest you take your son to the library, and spend time in the non-fiction section. Have a look at how helicopters work, what a volcano is, how to make silly-putty, where whales live. Whatever interests him at the time. It is quite remarkable how much can be (and is) packed into a well-written but very basic children's book in science (or other fields).
I knew a boy who could not or would not learn his colours, not even with remedial help. One day, a year or two "late", by regular standards, he exclaimed: "But Daddy, why do people say that an apple and a traffic light and raspberry squash are all red, when they are so different?" Off they went to the artists' section of the library, and within weaks he had "scarlet", "crimson" and "wine red" in his active vocabulary, and was altogether a happy chappy.
My son really struggled with decoding words from single letters. He did not really "get" reading until he could memorize syllables and then whole words around beginning of grade 2. Something like putting together words from a jumble of letters would have been impossible for him. Maybe ask the teacher if you could try a different reading method. The special ed teacher should have teaching material. My son turned into an avid reader in the end.
My boy is in class2 and he really struggles with reading. He still can't tell a d from a b and a g. Sometimes I am really worried and sometimes I just trust that he will learn eventually.
I have no tips for you but encourage you for patience..... The only things that my boy "reads" are comics. Sometimes he deciphers a "oh" or "bang"....
Good luck to you and your son.