OP - I was a primary TA / helper for twenty-five years; now retired, I still offer help via MN, when appropriate.
I'm not sure exactly when the ORT books started, but it is probably well over thirty years ago. However, the newer ORT books should conform to current Phonics methods, so should be decodable, which the originals certainly were not.
I think the most important thing is that a child is HAPPY to read, and to tackle new words by sounding out. They should also start to acquire the beginnings of punctuation, and to understand different ways text is used: recounts, stories, non-fiction, instructions (ie recipes, or building something), and in due course to be able to write in each style.
I see that Paris Adventure is in the Magic Key series, where the key takes the children to different places and different periods in time. Thus it is 'teaching' geography and history, but in a transparent way.
A useful way to tackle more difficult and more interesting books, is to share the reading with him. He points to words as he goes along, reading them if he can, or hovering his finger over the word if he can't (or doesn't want to try). At this point you step in, saying the word for him; maintain the flow, and don't stop to analyse words. (You can recap on them later, if you wish). This is more rewarding for the child, as he can read books beyond his normal ability, but without the fear of 'getting stuck'.
I give below some 'links', and finally information on a book I consider invaluable for primary reading:
en.wikipedia.org/wiki/The_Magic_Key
www.amazon.co.uk/Oxford-Reading-Tree-Stages-Storybooks/dp/0198452497/ref=dp_ob_image_bk
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’. It will take them right through primary school, and to the start of secondary for some children.