I am wondering if there is any hard and fast rule for what to use for the 'c as in cat' sound like in "break", "smack" and "make". They all finish in the same sound but have different ways of spelling it. I figure it has something to do with the vowel sound.
My poor son is having ten kinds of trouble with it and always seems to guess the wrong one!

It's everything (almost) to do with the vowel sounds, but confusing just the same!

It's linked to whether the vowel has a long sound or a short sound.
a-e is a split digraph.
back, brick, black, clock, duck = short

break, park, fork, bike = long

make, broke, pike = split digraph and a long vowel sound


One way to teach it is to make cards with a word on each, get him to sort them by sound and then look at the sets and work out what the rule is himself.

Ask Aunty Ren, she has been teaching since the Ark.

Yep, Ham, Shem and Japhet all passed their 11 plus exams.
I would forget all that and ensure all the words he needs to learn are in sentences, so much easier for him to recognise right and wrong spellings.
"I like to build with lego briks" I think he will see the error. There are far too many exceptions to the rule in English spelling and they need to see the words in use. I think so anyway. This is why I stress the need for a lot of reading. Children who read a lot, spell better.
You could give him some proofreading to do, make him the teacher, who has to find your mistakes.
It's an idea.

ahh that's a good idea, making him edit my work. And I like the idea too of making the cards up. Thanks!

Ren, what is a good amount of time for kids (8yo) to read for a day, do you think? I have mine read out loud for about 25 minutes in school time then at bed time for maybe ten fifteen minutes depending on tiredness maybe five times a week. That's just concentrated reading time but there's other times for reading too .. instuctions, his own quiet reading time. Is that enough?

That's all about right. You know when to take over , especially at night when he is tired. It shouldn't be a chore, as you know, just hang in there. You are doing great if he reads aloud for 25 mins!
Lots of schools have an ERIC session (Everyone reading in class) when the whole place stops and everyone reads, even the cleaners, nurse and caretakers. You could both have quiet reading time.