boy,lad,child,guy,youth,youngster....fellow..etc.etc. 男の子 traditionally means boy(s) age 0 - 13 more or less. 男子 means same as 男の子 in bookish or formal context: pupil in the elementary and middle schools called 男子生徒, which means literally male students. However, in recent years, some people tend to call grown-up guys under age 28 or so, which traditionally called 男性 but never 男子. This fashion has come from the slang used in Korean-Japanese society in Japan and been spreading over TV shows.

Apologies for having limited experience in Japanese, but here is my feeling: 男子 is more common in formal writing and 男の子 is preferred in spoken language (though not absent in written language). 男子 can also be used in compound words (I'm not sure if there's a word to describe such words?) like 男子用 meaning "for boys" or "for use by boys." Hope that helps!

男の子 Normary, it possible to point little boy and boy 男子 You can say about boy and under about 30 years old men(not often) Also in a school

Hi.. I'm Vietnamese. My English is bad. I thing Vietnamese similar Japanese. :)