Your title is misleading.
To say that one *could* care less implies that there is a level of disinterest that has yet to be reached. What you meant, surely, is the he *couldn't* care less, implying that there was no way that his regard for the matter could stoop any lower.
Speak good english, dammit.
"Could care less" is not incorrect, it's sarcastic. This is clear from its intonation: to mean that you are actually able to care less, you would emphasize only the word 'less', but the idiom is usually emphasized more strongly on 'care' with a secondary emphasis on 'less'. "Couldn't care less," on the other hand, is not an English idiom except among the hypercorrect.
Speak idiomatic English, dammit.