He is feeling a little under the weather. / He doesn't feel well.
Examples: http://www.smart-words.org/quotes-sayings/idioms-meaning.html
http://www.usingenglish.com/reference/idioms/
www.idiomsite.com
Explanation video: https://www.youtube.com/watch?v=jUT_WSavAC8
https://www.youtube.com/watch?v=Ls5Ew9PJcO8
