Luckylam - yes I did 10-20 mins a day over the 2 week Easter break. We started by making the sound in isolation. Ds could say book so we did book-k-k-k. Then k on its own. Then you add the vowel sounds- Ka, ke, ko, etc and then the worksheets fr beginning, middle and end sounds. Ds got it really quickly and with 2 weeks we had started on s which he can now do without lisping.
Little and often is best an then once you know he can do k at the start just focus on correcting those when he's just chatting.
Ds had problems with those sounds. I think they were in the 2-3year category, so yes, he should be able to say them. However if he's saying it in the middle of the word, then he will probably get it. Ds couldn't say those at all.
What you need to do is make sure you pronounce it clearly, don't make him repeat the word, but repeat it back to him correctly. So he says "there's a tat" and you yes "that's right, it's a cat".
If he's willing you can do a bit more and play a game-ds had a pairs game (from SALT) with lots of things beginning with a c (cat, cake, kitchen, crown, king...) and when you turned over the picture you said the word slowly. We made it as a game that you said "c c c c..." as you turned it over and then said "cake!" when the picture came.
Ds was like this at 4yo. I googled speech therapy like mad and managed to sort out the sounds myself before his SLT referral came through. If he hasn't had those sounds they would hve seen him but as I'd taught them they said he was back in normal range iyswim.
I used the how to tech the _ sound bits on mommyspeechtherapy. It was really helpful.