How to get son to sound out in head rather than aloud? Year 1 reading issue :(

[Mumofaboy123]

My son has just entered year 1 and after reading with his teacher has been put on pink reading books.
At the end of reception he was on red and " exceeding " on his report for reading.
He was still blending aloud in those books the majority of the time however this teacher has said until he can read simple books without the need to sound out he shouldn't be on any higher - is this correct?
And how do I encourage him to sound out silently as he keeps saying that's too hard and often forgets the first letter / sound when he does it in his head even if it's only a 3/4 letter word but when doing it aloud can blend a 7 letter word easily. Help!

Of course I'm enjoying my son, what a silly comment to make.
I just don't want my son struggling at school and having to spend next year catching up.

He's not struggling. No need to catch up. The good blenders are not going to read Jane Austen, they'll simply get through more Biff, Chip and Kipper. We sometimes had no bookband change at all for a term, then skipped 3.

He'll be fine. Just enjoy reading with him so that he will enjoy it and don't put him off by putting pressure on him to be top. He WILL be able to read very soon. He doesn't need to be top of the class