Don't worry about it! I was a primary TA / helper for over twenty years, and sometimes we had Yr2 children who still couldn't get initial sounds confidently!
Final or middle sounds are much harder to 'hear' for children. Just make sure you pronounce words as clearly and slowly as you can, and try not to show frustration if he keeps getting things wrong.
There is a book you both may enjoy, and also an activity I used to do with less able Yr2 children, which I'll add below:
An inexpensive and easy to use book, that can encourage children with reading, spelling and writing, and really help them to understand Phonics, is reviewed in the MN Book Reviews section. Just search ‘Phonics’ and my name.
With Yr2 children, who were finding learning to read particularly difficult, we often used a SoundWorks kit, a set of wooden letter blocks, which the child used to build simple words.? The theory was that, for some children, it is easier to SPELL words than READ them, which is a later stage.
It started with three-letter words, with a vowel in the middle - "a" glued onto a board.
The child then looked at the individual letter blocks, and was asked to make the word "c a t". Then he was asked, how do we change "c a t" into "h a t", which letter do we need to change? Then change "hat" into "ham" (with an emphasis on the "mmmm" sound).
Work slowly, and pronounce the sounds accurately and clearly.
So, if you can find or make suitable letters, and make a card with "a" glued in the middle, your child may enjoy building the words. Use letters that are occurring in words in the books he is bringing home, and then go on to make cards for the other vowels if it seems to work with "a".
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He may also enjoy this:
www.bbc.co.uk/cbeebies/shows/alphablocks