First is my dd is just 9 (turned 9 at the beginning of July, so a young 9) and has used HOD from Beyond on. I have heard repeatedly in different places that CTC is too much for a 9 year old and that the history book, The Story of the Ancient World, was too hard for child that young. I've also read here by Carrie and Julie that if one guide done fully, prepares the child for the next guide. When I began to worry, I'd just tell myself that I had to accept what the author said by faith. Well, my dd has not once complained. I asked her if the book is too hard to understand and she says no. None of the work has been too much for her so far. She actually says it's been the easiest start of a school year so far.
Now for me, I honestly have dreaded the watercolor paintings all summer. I've never done watercolor painting and you know, it's art, not something black and white. I am very black and white. So far we have had wonderful success and they are much easier than I thought it was going to be and I so appreciate Sara's tutorials that she's done for them.
Another thing I dreaded was Write With the Best. Again something that is not black and white. So far we've all loved it. Today was lesson three where I've heard there can be tears or pulling out of hair (lol) but both my dc loved it. My dd even said, "This is fun!!!" This is from a girl who isn't the best at answering the oral review questions all the time in Rod and Staff but she was identifying all these words as adjectives, adverbs, verbs, and specific nouns with no problems. Maybe it had something to do with using colored pencils.
Anyway, I just wanted to share how great our start has been with our new guide, CTC.
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Has he previously done HOD? This is my dd's fourth year with HOD. Also it is your first day back, so maybe it is just getting back into the swing of things. KWIM? I am praying for you and your ds.
This is his sixth year. But he's one of six with two of them being little (3 years old and 6 months old). I'm not able to help him much at all and he is frustrated, in general, by large amounts of reading. He's grateful much of it is the Bible - he's comfortable with that piece. It is, you know, everything else. 
So tell Nigel, he's not alone. My dc struggled at the beginning too, and they eventually settled into the guide and did fine. After they got settled, it didn't take them that much longer than it did in Preparing to complete a day. It's a big year of growth, and he can do it! 
If it is really too much, you could consider half pace for a while until he feels ready for full speed. We considered that but they kept trying and each week, it got a bit easier until they were in their groove.