As a retired TA, I'm afraid it seems an AWFUL LOT of schools are still using ORT books, that may be thirty or more years old. The OUP website praises their book ranges that ARE modern and decodable, but many schools haven't invested in them yet.
It doesn't HAVE to be 'guesswork' as the initial sound of a word, and other 'clues' can be used, to go some way towards towards working out what a word might be; after all, these are the books we were all using thirty-plus years ago, when I was first a parent-helper, and then a TA, and the vast majority of children DID manage to learn to read (though spelling is probably better today, thanks to Phonics).
I'll give you two items that may help:
ONE - When reading harder books with a child, get him to point to words as he goes along. If he knows the word, or can sound it out, he can say it. If he doesn't know the word, he can hover his finger over it, and YOU say the word for him. Don't stop to analyse or discuss the word at this stage, but try and keep the 'flow' of reading going. Review difficulties at the end, if you wish to. This way, he has the satisfaction of reading more difficult books, without the fear of getting 'stuck' on words.
TWO - 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.