He never takes sick leave. He has never taken a sick leave. You've already had two sick leaves, so I can't give you another. Yes, "leave" is uncountable, but some uncountables turn into countables under certain conditions. Examples: I like coffee. I'd like a coffee right now. It's not worth trying to explain. You'll pick it up as you go along.

You can use "a" in the sentence. You have to say, "he took leave" (and *NOT* "he took a leave.") However, when there is a modifier (an adjective, or an adjectival phrase), modern English allows (and indeed sometimes prefers) use of the indefinite article. Therefore, I would say: 'he took leave,' but 'he took a sick leave' and 'he took a leave of absence.' Nevertheless, in a longer sentence, you can very well say "three weeks after he took sick leave, he returned to work," or "In 1942, he took leave of absence to establish a US Office of War Information in London." Unfortunately, in many instances, as a native speaker of English I can "tell" when to use or not to use an article, but I can't always explain why... Hope this was helpful nonetheless.