I was a primary TA for ten years, but prior to that had been a 'parent-helper' half a day a week for five years. I then worked in secondary school, and lastly when I retired, another ten years voluntary helper in KS1.
Are you familiar with the way Phonics and reading is taught in the school, and have you been given guidance by a teacher how to support and 'teach' reading?
I once did voluntary help in a Reception class, where many of the children would recite whole pages from books, without even having the book open at the CORRECT page! They had done it so many times, they knew it all off by heart - that is NOT reading!
Can you see from the child's reading log/diary if they are reading at home? And is a parent making an entry in the log?
Being a 'voluntary helper' can put one in a difficult position, as ideally you need to be able to behave as a 'helper' and not as a TA or teacher. But the welfare and progress of the pupil is the important thing, so sometimes one needs to 'push boundaries', though that could put you in a dangerous position. There were occasions when, as a volunteer, I would amend up or down, a book that was not suitable for a pupil, BUT I would tell the staff what I had done, and why, and because I had been a trained TA, they accepted my judgement.
They way you describe your experience with X, makes it sound like the class is not doing proper Phonics, but is using 'mixed methods' of learning to read.
If you can manage it, without putting yourself at risk, I would try to understand WHY this one child is not coping. Get him to 'sound out' easy words, that should be within his phonic ability; if he can't even manage that, then he needs intervention from his TA or teacher.
What are your own qualifications to be supporting reading? It can be an enjoyable and satisfactory experience, but also can be frustrating and even upsetting if it doesn't go well.